Chemistry

Modern Periodic Table and Its Significance

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Modern Periodic Table and Its Significance

Modern Periodic Table :
Henry Moseley, an English physicist found that the atomic number (Z) was the fundamental property of an elements and not the atomic mass for classification of elements.
Modern Periodic Law :
‘‘Properties of elements are periodic functions of their atomic numbers, i.e., the number of protons or electrons present in the neutral atom of an element.’’
Long form of Periodic Table :
Arranged in increasing order of their atomic numbers.
The prediction of properties elements and their compounds can be made with precision. All drawbacks of Mendeleev’s Periodic Table vanish when the elements are arranged on the basis of increasing atomic numbers.
Elements in a Group :
(1)  They show similar chemical properties due to similar outer electronic configuration, i.e., same number of valence electrons.
(2)  They have gradation in properties due to gradually varying attraction of the nucleus and the outer valence electrons as we go down the group.
Main Features of the Long Form of the Periodic Table :
(1)  It shows arrangement of elements based on modern periodic law.
(2)  There are 18 vertical columns known as groups.
(3)  There are 7 horizontal rows known as periods.
(4)  Elements having similar outer electronic configurations, i.e., having same valence electrons have been placed in same groups, e.g.,

Group-1
KLMNOPQ
H(1)1
Li(3)2,1
Na(11)2,8,1
K(19)2,8,8,1
Rb(37)2,8,18,8,1
Cs(55)2,8,18,18,8,1
Fr(87)2,8,18,32,18,8,1

 

Group-2
KLMNOPQ
Be(4)22
Mg(12)2,8,2
Ca(20)2,8,8,2
Sr(38)2,8,18,8,2
Ba(56)2,8,18,18,8,2
Ra(88)2,8,18,32,18,8,2

 

Group-13
KLMNOP
B(5)23
Al(13)2,8,3
Ga(31)2,8,18,3
In(49)2,8,18,18,3
Tl(81)2,8,18,32,18,3
Group-14
KLMNOP
C(6)24
Si(14)2,8,4
Ge(32)2,8,18,4
Sn(50)2,8,18,18,4
Pb(82)2,8,18,32,18,4

 

Group-15
KLMNOP
N(7)25
P(15)2,8,5
As(33)2,8,18,5
Sb(51)2,8,18,18,5
Bi(83)2,8,18,32,18,5
Group-16
KLMNOP
O(8)26
S(16)2,8,6
Se(34)2,8,18,6
Te(52)2,8,18,18,6
Po(84)2,8,18,32,18,6
Group-17
KLMNOP
F(9)27
Cl(17)2,8,7
Br(35)2,8,18,7
I(53)2,8,18,18,7
At(85)2,8,18,32,18,7
Group-18
KLMNOP
He(2)2
Ne(10)2,8
Ar(18)2,8,8
Kr(36)2,8,18,8
Xe(54)2,8,18,18,8
Rn(86)2,8,18,32,18,8

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(5) In periods, elements in which the number of electrons in the outermost shell increases gradually in step one are placed, e.g.,

Period 1

(K-shell)

H

1

He

2

Second Period

(K, L, shells)

Li(3)

2,  1

Be(4)

2, 2

B(5)

2, 3

C(6)

2, 4

N(7)

2, 5

O(8)

2, 6

F(9)

2, 7

Ne(10)

2, 8

(6)  Each group in the table signifies identical outer shell electronic configuration i.e., same valence electrons, e.g., group 1 has 1 valence electron, group 2 has 2 valence electrons, group 13 has 3, group 14 has 4 valence electrons.
(7)  Each period starts with filling of new shell, e.g.,

1st Period    –    K shell (1st shell) starts filling with Hydrogen and ends at Helium.

2nd Period    –    L shell (2nd shell) starts filling from Li (3) upto Ne (10)

3rd Period    –    M shell (3rd shell) start filling from Na (11) upto Ar (18)

4th Period    –    N shell (4th shell) starts filling from K (19) upto Kr (36) and so on.

(8)  The periodic table is divided in four blocks :
(a)  s-block elements :     Group 1 and 2 elements are called s-block elements.
(b)  p-block elements : Group 13 to 18 elements are called p-block elements
(c)  d-block elements : Group 3 to group 12 are called d-block elements or transition elements (in between s- block and p-block elements)
(d)  f-block elements : The elements placed at the bottom of the periodic table are known as f-block elements. The fourteen elements after La(57) (Lanthanum) are called Lanthanoides and 14 elements after Actinium Ac (89) are called Actinoides.

Modern Periodic Table and Its Significance 1

Why does Diffusion take place

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Why does Diffusion take place

Diffusion : The spreading out and mixing of a substance with another substance due to the motion of its particles is called diffusion.
Diffusion is a property of matter which is based on the motion of its particles.
Diffusion is fastest in gases because the particles in gases move very rapidly. The diffusion is slowest in solids because the particles in solids do not move much.
The rate of diffusion increases on increasing the temperature of the diffusing substance. This is because when the temperature of a substance is increased by heating, its particles gain kinetic energy and move more rapidly and this increase in the speed of the particles of a substance increases the rate of diffusion.
Why does Diffusion take place 1Diffusion in gases
Diffusion in gases is very fast. This is because the particles in gases move very quickly in all directions.
Ex.       When we light an incense stick (agarbatti) in a corner of our room, its fragrance spreads in the whole room very quickly. The fragrance of burning incense stick spreads all around due to the diffusion of its smoke into the air.
Ex.       When someone opens a bottle of perfume in one corner of a room, its smell spreads in the whole room quickly. The smell of perfume spreads due to the diffusion of perfume vapours into air.

Diffusion in liquids
Diffusion in liquids is slower than that in gases. This is because the particles in liquids move slower as compared to the particles in gases.
Ex.     The spreading of purple colour of potassium permanganate into water, on its own, is due to the diffusion of potassium permanaganate particles into water
Ex.      The spreading of blue colour of copper sulphate into water, on its own, is due to the diffusion of copper sulphate particles into water.
The rate of diffustion in liquids is much faster than that in solids because the patricles in a liquid move much more freely, and have greater spaces between them as compared to particles in the solids.

Diffusion in solids
Diffusion in solids in a very, very slow process.
Ex.      If we write something on a blackboard and leave it uncleaned for a considerable period of time we will find that it becomes quite difficult to clean the blackboard afterwards. This is due to the fact that some of the a particles of chalk have diffused into the surface of blackboard.
Ex.     If two metal blocks are bound together tightly and kept undisturbed for a few years, then the  particles of one metal are found to have diffused into the other metal.

What is the definition of an acid and a base?

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What is the definition of an acid and a base?

 

What are acids?

  • The Swedish chemist, Arrhenius proposed the following definition of an acid.
    An acid is a substance which ionises or dissociates in water to produce hydrogen ions, H+.
  • For example, hydrochloric acid, HCl(aq) is a solution of hydrogen chloride in water, obtained by dissolving pure hydrogen chloride gas in water.
    HCl(g) → HCl(aq)
    As the gas dissolves in water, the hydrogen chloride molecule reacts with water and ionises to produce hydrogen ion, H+.
    HCl(aq) → H+(aq) + Cl–(aq)
    The hydrogen ion then attaches itself to a water molecule to form the hydroxonium ion, H3O+.
    H+(aq) + H2O(l) → H3O+(aq)
    Hence, the overall equation for the ionisation of hydrogen chloride is given below.
    HCl(aq) + H2O(1) → H3O+(aq) + Cl–(aq)
    What is the definition of an acid and a base 1
  • For the sake of convenience, the term ‘hydrogen ion is used to replace ‘hydroxonium ion and H+(aq) is used in place of H3O+(aq).
  • Hence, the ionisation of hydrochloric acid in water can be represented as:
    What is the definition of an acid and a base 2
  • Other acids ionise similarly in water. Examples:
    What is the definition of an acid and a base 3
    What is the definition of an acid and a base 4
  • Hydrochloric acid is known as a monoprotic acid. This acid contains only one ionisable hydrogen atom, producing only one hydrogen ion (proton) per molecule of acid.
  • Polyprotic acids can produce more than one hydrogen ion per molecule of acid. Sulphuric acid is a diprotic acid, whereas phosphoric acid is a triprotic acid.
  • Basicity of an acid is the number of ionisable hydrogen atoms per acid molecule.
  • A number of non-metal oxides react with water to produce acidic solutions which contain hydrogen ions and turn blue litmus paper red. They are called acidic oxides.
    (a) Carbon dioxide reacts with water to form carbonic acid.
    CO2(g) + H2O(l) → H2CO3(aq)
    (b) Sulphur trioxide reacts with water to form sulphuric acid.
    SO3(g) + H2O(l) → H2SO4(aq)
    (c) Dinitrogen pentoxide reacts with water to form nitric acid.
    N2O5(g) + H2O(l) → 2HNO3(aq)
  • Not all non-metal oxides are acidic oxides. Only those that are able to react with water can produce acidic solutions. For example, carbon monoxide does not react with water. Therefore, carbon monoxide is classified as a neutral oxide.
  • Acids are classified into two groups, mineral acids and organic acids.
  • Mineral acids are obtained frdm minerals, whereas organic acids are extracted from animal and plant materials.

Table 1 and Table 2 show some examples of mineral and organic acids.

Mineral acid
NameFormula
Carbonic acidH2CO3
Hydrochloric acidHCl
Hydrochlorous acidHClO
Nitrous acidHNO2
Nitric acidHNO3
Sulphurous acidH2SO3
Sulphuric acidH2SO4
Phosphoric acidH3PO4

 

Organic acid
NameFormula
Methanoic acidHCOOH
Ethanoic acidCH3COOH
Propanoic acidC2H5COOH
Ascorbic acidC6H8O5
Citric acidC6H6O7
Lactic acidC3H6O3
Malic acidC4H6O5
Ethanedioic acidH2C2O4

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What are bases?

  1. Arrhenius’ definition of a base:
    A base is a substance which ionises in water to produce ydroxide ions, OH–.
  2. Bases are divided into two categories, ionic bases and covalent bases.
  3. Ionic bases consist of metal oxides and metal hydroxides. Metal oxides contain oxide ions, O2- and metal hydroxides contain hydroxide ions, OH–.
  4. When a soluble metal hydroxide such as sodium hydroxide dissolves in water, it ionises to release the hydroxide ion.
    What is the definition of an acid and a base 5
  5. When a soluble metal oxide dissolves in water, it reacts with water to form the hydroxide ion as one of its products. For example, calcium oxide reacts with water to form calcium hydroxide, which then ionises to produce hydroxide ion.
    What is the definition of an acid and a base 6
  6. Insoluble metal oxides and metal hydroxides are classified as bases because they satisfy the alternative definition of a base.
    A base is a substance that reacts with an acid to form a salt and water only.
  7. (a) For example, magnesium hydroxide reacts with hydrochloric acid to form the salt magnesium chloride and water.
    What is the definition of an acid and a base 7
    (b) On the other hand, copper(II) ‘oxide reacts with nitric acid to produce the salt copper(II) nitrate and water.
    What is the definition of an acid and a base 8
  8. The most common covalent base is ammonia, NH3. Ammonia solution is obtained by dissolving pure ammonia gas in water. When ammonia gas dissolves in water, it reacts with water to produce hydroxide ion.
    What is the definition of an acid and a base 9
    Notice that the ammonia, NH3 molecule has accepted a proton, H+ from water to form the ammonium ion, NH4+.
  9. Bases that are soluble in water are called akalis (Figure).
    What is the definition of an acid and a base 10
  10. An alkali is defined as the following.
    An alkali is a base that is soluble in water and ionises to produce hydroxide ions.
  11. According to this definition, ammonia can be classified as an alkali.

Table shows some examples of bases and alkalis.

Soluble base (alkali)Insoluble base
NameFormulaNameFormula
AmmoniaNH3Magnesium oxideMgO
Sodium oxideNa2OMagnesium hydroxideMg(OH)2
Sodium hydroxideNaOHAluminium oxideAl2O3
Potassium oxideK2OAluminium hydroxideAl(OH)3
Potassium hydroxideKOHZinc oxideZnO
Calcium oxideCaOZinc hydroxideZn(OH)2
Calcium hydroxideCa(OH)2Copper(II) oxideCuO
Barium oxideBaOCopper(II) hydroxideCu(OH)2
Barium hydroxideBa(OH)2Lead(II) oxidePbO

Properties of Ionic and Covalent Compounds

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Properties of Ionic and Covalent Compounds

  • Ionic and covalent compounds differ in their properties because the particles in each of these two compounds are held together by different types of chemical bonds.
  • Table compares and contrasts the properties of ionic and covalent compounds.
    Covalent compounds Ionic compounds (composed of simple molecules)
    (a) Have high melting and boiling points(a) Have low melting and boiling points
    (b) Exist as solids at room temperature.
    Non-volatile

    (b) Usually exist as liquids or gases at room temperature.
    Volatile

    (c) Conduct electricity in the molten state or in an aqueous solution but do not conduct electricity in the solid state

    		(c) Do not conduct electricity in the solid and liquid states
    (d) Usually soluble in water but insoluble in organic solvents such as ether, alcohol, benzene, tetrachloromethane, propanone and other

    (d) Usually insoluble in water but soluble in organic solvents such as ether, alcohol, benzene, tetrachloromethane, propanone and other

Explaining the melting and boiling points of ionic compounds

    • Table shows the melting and boiling points of some ionic compounds.
      Ionic compoundMelting point (°C)Boiling point (°C)
      Calcium oxide, CaO25802850
      Magnesium chloride, MgCl27141412
      Sodium fluoride, NaF9931695
      Aluminium oxide, Al2O320302970
      Sodium chloride, NaCl8011420
    • The melting and boiling points of ionic compounds are high.
  • The high melting and boiling points of ionic compounds can be explained as below:
    • Ionic compounds are composed of oppositely-charged ions (positive and negative ions) arranged in a three-dimensional giant crystal lattice.
    • The oppositely-charged ions are held together by strong electrostatic forces of attraction, known as ionic bonds.
    • A lot of heat energy is needed to break the strong ionic bonds during melting or boiling.
    • Hence, ionic compounds have high melting and boiling points with low volatility.

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Explaining the melting and boiling points of covalent compounds

  • Covalent compounds are composed of molecules.
  • The bonding in these covalent compounds consists of
    • strong covalent bonds between the atoms in the molecule.
    • weak forces of attraction between the molecules.
  • An example is shown in Figure. It shows the two types of bonds in liquid tetrachloromethane, CCl4.
    Properties of Ionic and Covalent Compounds 1

    • Table shows the melting and boiling points of four covalent compounds.
      Covalent compoundMelting point
      (°C)

      Boiling point
      (°C)

      Ethanol, C2H5OH-11778
      Tetrachloromethane, CCl4-2376.8
      Ammonia, NH3-78-33
      Methane, CH4-182-164
    • The melting and boiling points of covalent compounds are low.
  • The low melting and boiling points of covalent compounds can be explained as below:
    • In a covalent compound, the covalent molecules are held together by weak forces of attraction.
    • A small amount of heat energy is required to overcome the weak intermolecular forces of attraction during melting or boiling.
    • Hence, the covalent compound has low melting and boiling points with high volatility.

Explaining the electrical conductivity of ionic compounds

  • The electrical conductivity of ionic compounds in the solid state can be explained as below:
    • Ionic compounds are composed of oppositely-charged ions.
    • In the solid state, the positive and negative ions are locked in fixed positions and cannot move freely.
    • Hence, ionic compounds cannot conduct electricity in the solid state.
  • The electrical conductivity of ionic compounds in the molten (liquid) and aqueous states can be explained below:
    • When the ionic compounds are melted through heating or dissolved in water, the positive and negative ions will break free and become mobile, that is able to move freely.
    • The presence of free mobile ions enable ionic compounds to conduct electricity in the molten or aqueous states.

Explaining the electrical conductivity of covalent compounds

  • Table shows the electrical conductivity of a few covalent compounds.
    Covalent compoundElectrical conductivity
    SolidLiquid
    GlucoseNon­ conductorNon­ conductor
    AcetamideNon­ conductorNon­ conductor
    NapthaleneNon­ conductorNon­ conductor
    TetrachloromethaneNon­ conductorNon­ conductor
  • The electrical conductivity of covalent compounds in the solid and liquid states can be explained as below:
    • Covalent compounds are composed of simple covalent molecules in the solid and liquid states.
    • There are no free mobile ions in these two states.
    • Hence, covalent compounds cannot conduct electricity in the solid and liquid states.

Explaining the solubility of ionic compounds

  • The solubility of ionic compounds in water can be explained as below:
    • Ionic compounds are composed of ions.
    • The ions are easily hydrated by water molecules to form hydrated ions.
    • The hydration of ions by water molecules liberates heat energy.
    • As a result, ionic compounds are usually soluble in water.
  • The solubility of ionic compounds in organic solvents can be explained as below:
    • Organic solvents such as ether, alcohol, benzene and tetrachloromethane consist of covalent molecules which cannot hydrate ions.
    • As a result, ionic compounds are insoluble in organic solvents.

Explaining the solubility of covalent compounds

  • The solubility of covalent compounds in water can be explained as below:
    • Covalent compounds consist of covalent molecules.
    • Water cannot hydrate covalent molecules.
    • Hence, covalent compounds are usually insoluble in water.
  • The solubility of covalent compounds in organic solvents can be explained as below:
    • Covalent molecules in covalent compounds and organic molecules in organic solvents are both held together by weak intermolecular forces of attraction.
    • As a result, the covalent molecules in the covalent compounds are easily miscible with the organic molecules in the organic solvents because they have the same type of weak intermolecular forces of attraction.
    • Hence, covalent compounds are usually soluble in organic solvents.

Properties of Ionic and Covalent Compounds Experiment

Aim: To compare the properties of ionic and covalent compounds.
Materials: Magnesium chloride crystals, sodium sulphate crystals, solid lead(II) bromide, diethyl ether, hexane, cyclohexane, distilled water and naphthalene.
Apparatus: Watch glasses, dropper, test tubes, crucible, battery, bulb, switch, Bunsen burner, tripod stand, carbon electrodes, pipe-clay triangles, connecting wires with crocodile clips and beaker.
Procedure:
A. Melting and boiling points

  1. Half spatula of magnesium chloride crystals and sodium sulphate crystals are placed separately in two different watch glasses. The physical state of each substance is recorded.
  2. Three drops of diethyl ether and hexane are placed separately in two different watch glasses. The physical state of each substance is recorded.
  3. All the watch glasses are left aside for 5 to 10 minutes. All the changes are recorded.
  4. Inferences regarding their volatility, melting and boiling points are made based on the observation.

B. Solubility in water and organic solvents

  1. A quarter spatula of magnesium chloride crystals are placed in a test tube.
  2. 5 cm3 of distilled water is added to the test tube.
  3. The mixture in the test tube is shaken well.
  4. All the changes are recorded.
  5. Steps 1 to 4 are repeated using liquid cyclohexane to replace distilled water.
  6. Steps 1 to 5 are repeated using 5 cm3 of diethyl ether to replace the magnesium chloride crystals.

C. Electrical conductivity

  1. A crucible is filled with solid lead(II) bromide until it is half full.
  2. The apparatus as shown in Figure is set up.
    Properties of Ionic and Covalent Compounds 2
  3. The switch is turned on. The observation on whether the bulb glows and the changes at the electrodes (if any) are made.
  4. The switch is then turned off. The solid lead(II) bromide in the crucible is heated until it melts completely.
  5. The switch is turned on again. The observation on whether the bulb glows and the changes at the electrodes (if any) are made.
  6. Steps 1 to 5 are repeated using solid naphthalene to replace solid lead(II) bromide.
  7. Another test on the electrical conductivity of aqueous magnesium chloride solution is carried out by setting up the apparatus as shown in Figure. Observation on whether the bulb glows and the changes at the electrodes (if any) are recorded.
    Properties of Ionic and Covalent Compounds 3

Results:

A. Melting and boiling points

SubstanceObservationInferences
Magnesium chloride crystalsThe substance remains as a white solid even after 10 minutes.Magnesium chloride has high melting and boiling points. It is non-volatile.
Sodium sulphate crystalsThe substance remains as a white solid even after 10 minutes.Sodium sulphate has high melting and boiling points. It is non-volatile.
Diethyl etherThe colourless liquid disappears/vaporises and the watch glass becomes dry after 10 minutes.Diethyl ether has low melting and boiling points. It is volatile.
HexaneThe colourless liquid disappears/vaporises and the watch glass becomes dry after 10 minutes.Hexane has low melting and boiling points. It is volatile.

B. Solubility in water and organic solvents

SubstanceObservationInferences
Solubility in waterSolubility in cyclohexane
Magnesium chlorideThe white solid dissolves in water to form a colourless solution.The white solid does not dissolve in cyclohexane.Magnesium chloride is soluble in water but insoluble in cyclohexane.
Diethyl etherTwo layers of colourless liquids are formed.The colourless liquid dissolves in cyclohexane to form a colourless solution.Diethyl ether is insoluble in water but soluble in cyclohexane.

C. Electrical conductivity

SubstanceState of substanceObservationInferences
The bulbChanges at the carbon electrodes

Lead(II) bromide

SolidThe bulb does not glow.No changeLead(II) bromide cannot conduct electricity in the solid state but can conduct electricity in the liquid state.
Liquid/moltenThe bulb glows brightly.A reddish-brown vapour is liberated at one of the electrodes.
NaphthaleneSolidThe bulb does not glow.No changeNaphthalene cannot conduct electricity in the solid and liquid states.
Liquid/moltenThe bulb does not glow.No change
Magnesium chlorideAqueous solutionThe bulb glows brightly.Bubbles of gas are liberated at both the carbon electrodes.Magnesium chloride can conduct electricity in the aqueous solution.

Discussion:

  1. Magnesium chloride crystals and sodium sulphate crystals are ionic compounds. They are made up of positive and negative ions which are attracted together by strong ionic bonds. A lot of heat energy is needed to overcome these bonds during melting or boiling. Hence, they have high melting and boiling points and are non-volatile.
  2. Diethyl ether and hexane are covalent compounds. They consist of molecules that are attracted to each other by weak intermolecular forces. Little heat energy is needed to overcome these weak forces during melting or boiling. Hence, they have low melting and boiling points and are volatile.
  3. Magnesium chloride, as an ionic compound, is
    • soluble in water, but
    • insoluble in cyclohexane (organic solvent).
  4. Diethyl ether, as a covalent compound, is insoluble in water, but soluble in cyclohexane (organic solvent).
  5. In solid lead(II) bromide (an ionic compound), the lead(II) ions and bromide ions are closely packed at fixed positions in an orderly manner. Hence, the ions do not move freely. As a result, solid lead(II) bromide cannot conduct electricity.
  6. In molten lead(II) bromide, the lead(II) ions and bromide ions are mobile or can move freely. Hence, molten lead(II) bromide can conduct electricity.
  7. Magnesium chloride (an ionic compound) ionises completely in an aqueous solution to become free mobile magnesium ions and chloride ions. Hence, an aqueous solution of magnesium chloride can conduct electricity.
    Properties of Ionic and Covalent Compounds 4
  8. Naphthalene, as a covalent compound, is made up of covalent molecules only. Hence, it cannot conduct electricity in the solid and liquid states.

Conclusion:

  1. Ionic compounds are non-volatile and have high melting and boiling points. They are usually soluble in water but insoluble in organic solvents. They can conduct electricity in the molten and aqueous states.
  2. Covalent compounds are volatile and have low melting and boiling points. They are usually insoluble in water but soluble in organic solvents. They cannot conduct electricity in the solid and liquid states.

Types of covalent molecules

  1. There are two types of covalent molecules.
    (a) Simple molecules such as water, carbon dioxide, ammonia and tetrachloromethane.
    (b) Macromolecules or giant molecules such as silicon dioxide and diamond.
  2. Figure shows the structures of diamond and silicon dioxide.
    Properties of Ionic and Covalent Compounds 5
  3. In a macromolecule, all the atoms are bonded to each other by covalent bonds in a giant lattice structure.
  4. These macromolecules
    (a) have high melting and boiling points because a lot of heat energy is needed to break the strong covalent bonds in the giant lattice structure.
    (b) cannot conduct electricity because they do not have free mobile ions.
    (c) are insoluble in water.

Uses of covalent compounds as solvents

  1. Many covalent compounds have low melting and boiling points. Hence, they exist as liquids at room temperature and are volatile.
  2. Covalent compounds in the form of liquids are mostly used as solvents in our daily life. Most of these liquids are organic compounds. They are known as organic solvents.
  3. Examples of some common organic solvents are alcohols such as ethanol, ethers such as dimethyl dimethyl ether, propanone, chloroform (trichloromethane), turpentine and petrol.
  4. Organic solvents are used
    (a) as solvents to prepare solutions.
    (b) to remove and clean dirt on surfaces which cannot be removed by water.
  5. Table lists out the uses of some organic solvents.
    SolventsUses
    TurpentineTo dissolve paint
    Petrol and keroseneAs solvents to remove greasy or oil dirts
    Alcohols, propanone and turpentineAs solvents to prepare varnish, shellac and lacquer
    AlcoholsAs solvents in medicine such as iodine solution
    EthersAs solvents in the extraction of chemicals from aqueous solutions
    PropanoneTo remove nail varnish

    Chlorofluorocarbons
    (CFC)

    		As solvents to clean computer circuit boards
    Alcohols and ethersAs solvents for ink and dyes
    Alcohols, ethers and propanoneAs volatile solvents in the preparation of cosmetic products such as perfumes
  6. Most organic solvents such as benzene, chloroform and propanone are poisonous and harmful.

How do you Calculate the Molar Mass of a Substance?

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How do you Calculate the Molar Mass of a Substance?

  1. Molar mass of a substance is the mass of one mole of the substance in grams. It has a unit of grams per mole (g mol-1).
  2. One mole of any substance contains 6.02 × 1023 particles. Therefore, the molar mass of a substance contains 6.02 × 1023 particles of the substance.
  3. The molar mass of any substance is numerically equal to its relative atomic, molecular or formula mass.
    (a) This means, to measure 1 mole of atoms of any element, we only need to weigh a mass equal to its relative atomic mass in grams.
    (b) Similarly, the mass of 1 mole of molecules of any molecular substance is equal to its relative molecular mass in grams.
    (c) The mass of 1 mole of any ionic substance is equal to its relative formula mass in grams.
  4. Based on Table below, 1 mole of magnesium can be measured by weighing 24 g of magnesium.
    This amount of magnesium contains 6.02 × 1023 magnesium atoms.

    SubstanceRelative massMolar mass
    (g mol-1)
    Magnesium, MgAr = 2424
    Helium, HeAr = 44
    Hydrogen gas, H2Mr = 2(1) = 22
    Methane, CH4Mr = 12 + 4(1) = 1616
    Sodium chloride, NaClFr = 23 + 35.5 = 58.558.5
    Zinc bromide, ZnBr2Fr = 65 + 2(80) = 225225

    [Relative atomic mass: H, 1; He, 4; C, 12; Na, 23; g Mg, 24; Cl, 35.5; Zn, 65; Br, 80]

  5. The mass of any number of moles of a substance or vice versa can be calculated easily using the following relationship.
    How do you Calculate the Molar Mass of a Substance 1
  6. Equal numbers of moles of substances always contain the same number of particles even though they differ in mass and size.
    How do you Calculate the Molar Mass of a Substance 2
    Figure: Both blocks of magnesium and iron contain 0.5 x 6.02 x 1023 atoms. [Relative atomic mass: Mg, 24; Fe,56]
  7. For this reason, we can compare the number of particles in substances by comparing the number of moles of the substances.

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Molar Mass of a Substance Problems with Solutions

1. Find the mass of the following.
(a) 0.5 mole of copper
(b) 1.5 moles of carbon dioxide
[Relative atomic mass: C, 12; O, 16; Cu, 64]
Solution:
(a) Relative atomic mass of copper = 64
So, the molar mass of copper = 64 g mol-1
Mass of 0.5 mole of copper = number of moles x molar mass
= 0.5 x 64 = 32g
(b) Relative molecular mass of carbon dioxide, CO2 = 12 + 2(16) = 44
So, the molar mass of CO2 = 44 g mol-1
Mass of 1.5 moles of CO2 = number of moles x molar mass
= 1.5 x 44 = 66g

2. Find the number of moles of sodium hydroxide with the mass of 2.0 g.
[Relative atomic mass: H, 1; O,16; Na, 23]
Solution:
Relative formula mass of sodium hydroxide, NaOH = 23+16+1 = 40
So, the molar mass of NaOH = 40 g mol-1
Therefore, the number of moles of NaOH
= mass of NaOH ÷ molar mass
= 2/40
= 0.05 mol

3. What is the number of moles of calcium nitrate in 49.2 g of calcium nitrate, Ca(NO3)2?
[Relative atomic mass: O, 16; Ca, 40; N, 14]
A. 0.30 mol
B. 0.36 mol
C. 0.45 mol
D. 0.48 mol
Solution:
Relative formula mass of Ca(NO3)2 = 40 + 2[14 + 3(16)] = 164
So, molar mass of Ca(NO3)2 = 164 g mol-1
Number of moles of Ca(NO3)2 = Mass of Ca(NO3)2 ÷ molar mass of Ca(NO3)2
= 49.2/164
= 0.3 mol
Answer: A

4. What is the mass of gold that has the same number of atoms as in 4 g of oxygen?
[Relative atomic mass: O, 16; Au, 197]
Solution:
Number of moles of oxygen atoms, O = mass of O ÷ molar mass of O
= 4/16 = 0.25 mol
0.25 mole of gold will have the same number of atoms with 0.25 mole of oxygen.
Therefore, the mass of gold, Au
= number of moles of Au x molar mass of Au
= 0.25 x 197
= 49.25 g

5. What is the mass of hydrogen gas that has twice the number of molecules as in 1.6 g of oxygen gas?
[Relative atomic mass: H, 1; O,16]
Solution:
Relative molecular mass of oxygen gas, O2 = 2(16) = 32
Therefore, the molar mass of O, = 32 g mol-1
Number of moles of O2 = mass of O2 ÷ molar mass of O2
= 1.6/32 = 0.05 mol
Thus, the number of moles of hydrogen gas, H2 that contains twice the number of molecules in 0.05 mole of O2
= 2 x 0.05 mol = 0.1 mol
Mass of 0.1 mole of H2
= number of moles of H2 x molar mass of H2
= 0.1 x 2(1) = 0.2 g

6. How many times more is the number of atoms in 7 g of nitrogen gas compared to that in 7 g of iron? [Relative atomic mass: N, 14; Fe, 56]
Solution:
Here, we need to compare the number of moles of atoms in both substances.
How do you Calculate the Molar Mass of a Substance 3
Therefore, the number of atoms in 7 g of nitrogen gas is 4 times more than that in 7 g of iron.