Laws of Chemical Combination

The substances react according to certain laws which are called laws of chemical combination.

By Sakshi Goel | 28 Oct'18 | 1 K Views |

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Theory

The laws of chemical combination are:

1. Law of conservation of mass:
Proposed by Antoine Lavoisier, this law states that:
In a chemical combination, matter is neither created nor destroyed.
It means that the total mass of the substances involved in the reaction does not change. The total mass of the reactants is equal to the total mass of the products. Suppose 4.4 g of CaO and 5.6 g of CO₂ is reacted, it will give 10.0 g of CaCO₃.
$CaO + {CO}_{2} â†’ {CaCO}_{3} 4.4 g 5.6 g 10.0 g$
The combined mass of reactants is exactly equal to the mass of the product.

2. Law of definite proportions or Law of constant compositions:
Proposed by Joseph Proust, this law states that:
A chemical compound always contains exactly the same proportion of elements by mass, irrespective of the source.
For example, water sample collected from anywhere will always contain definite proportions of hydrogen and oxygen. 18.0 g of water contains 16.0 g of oxygen and 2.0 g of hydrogen.
$M a s s r a t i o = \frac{2.0 g o f h y d r o g e n}{16.0 g o f o x y g e n} = 1 : 8$
So, the ratio of 1:8 remains same for every water sample.

3. Law of multiple proportions:
Published by John Dalton, this law states that:
When elements combine to form two or more than two compounds, the masses of one of the elements which combine with the fixed mass of the other, bear a simple whole number ratio.
For example, carbon and oxygen combine to form CO and CO₂. In CO, 12 parts by mass of carbon combine with 16 parts by mass of oxygen. In CO₂, 12 parts by mass of carbon combine with 32 parts by mass of oxygen. Ratio of masses of oxygen is 16:32 or 1:2, which is a simple whole number ratio.

MCQ

Problem:
Which of the following pairs of substances illustrate the law of multiple proportions?
(a) MgO and Mg(OH)₂
(b) H₂O and D₂O
(c) NaCl and NaBr
(d) CO and CO₂

Solution:

In CO, 12 parts of carbon combines with 16 parts of oxygen.
In CO₂, 12 parts of carbon combines with 32 parts of oxygen.
So, the ration of oxygen in both the compounds is 1:2. This illustrates law of multiple proportions.
Correct option is (d).

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Problem:
Law of definite proportions does not apply to nitrogen oxide because
(a) Atomic weight of N is not constant
(b) Molecular weight of nitrogen is variable
(c) Atomic weight of oxygen is variable
(d) Nitrogen has different valencies in different oxides

Solution:

In different oxides of nitrogen, the valencies of nitrogen varies from +1 to +5. So, law of definite proportions does not apply to oxides of nitrogen.
Option (d) is correct.

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Problem:
Irrespective of the source, pure sample of water always yields 88.89 % mass of oxygen and 11.11% mass of hydrogen. This is explained by the law of
(a) Conservation of mass
(b) Multiple proportions
(c) Definite proportions
(d) Constant volume

Solution:

This illustrates law of definite proportions.
Correct option is (c).

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Problem:
Carbon forms two oxides containing 42.9 % and 27.3 % carbons respectively. Which law of chemical combination does this illustrate?

Solution:

In 100 g of first oxide, carbon is 42.9 g and oxygen is (100-42.9) = 57.1 g
So, 42.9 parts of carbon contains 57.1 g of oxygen. The ratio of oxygen:carbon = 57.1:42.9 or 1.33:1
Similarly, in 100 g of 2nd oxide, carbon is 27.3 g and oxygen is (100-27.3) = 72.7 g
So, 27.3 parts of carbon contains 72.7 g of oxygen. The ratio of oxygen:carbon = 72.7:27.3 or 2.66:1
Ratio of masses of oxygen in both oxides is 1.33:2.66 or 1:2.
This illustrates law of multiple proportions.
Correct option is (b).

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Problem:
3 g of a hydrocarbon on combustion with 11.2 g of oxygen produces 8.8 g of CO₂ and 5.4 g of H₂O.The data illustrates the law of
(a) Conservation of mass
(b) Multiple proportions
(c) Definite proportions
(d) Avogadros law

Solution:

Masses of hydrocarbon and oxygen = 3 g + 11.2 g = 14.2 g
Masses of CO₂and H₂O = 8.8 g + 5.4 g = 14.2 g
Masses of reactants = masses of products
So, law of conservation of mass is illustrated. Correct option is (a).

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Problem:
Hydrogen combines with chlorine to form HCl. It also combines with sodium to form NaH. If sodium and chlorine also combine with each other, they will do so in the ratio of their masses as
(a) 23:35.5
(b) 35.5:23
(c) 1:1
(d) 23:1

Solution:

Atomic masses of H, Na and Cl is 1, 23 and 35.5 g/mol respectively.
In HCl, 1 part of H combines with 35.5 parts of Cl.
In NaH, 23 parts of Na combines with 1 part of H.
The ratio of H in both compounds is 1:1.
According to law of multiple proportions, Na and Cl will combine in the ratio of their masses as 23:35.5.
So, correct option is (a).

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Problem:
Zinc sulphate contains 22.65% Zn and 43.9% H₂O. If the law of constant proportions is true, then the mass of Zinc required to give 40 g crystal will be
(a) 90.6 g
(b) 9.06 g
(c) 0.906 g
(d) 906 g

Solution:

As each crystal of zinc sulphate will contains 22.65% Zn, so 40 g crystal will also contain 22.65% Zn.
Zn in the crystal = 22.65% of 40 g
=(22.65/100) x 40 = 9.06 g
Correct option is (b).

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Problem:
Phosphorus forms two oxides P₂O₃ and P₂O₅. Which law can be proved by the weights of P and O?
(a) Constant composition
(b) Multiple proportions
(c) Gay Lussac law
(d) Conservation of mass

Solution:

The law of multiple proportions can be proved by the weights of P and O in these two compounds.
Atomic mass of P is 31 g/mol
Atomic mass of O is 16 g/mol.
In P₂O₃, 62 parts of P combines with 48 parts of O.
In P₂O₅, 62 parts of P combines with 80 parts of O.
The ratio of O in both compounds is 48:80 or 3:5 which is a simple whole number ratio.
This proves the law of multiple proportions.
So, correct option is (b).

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