Law of Constant Proportions: Overview, Questions, Preparation

The Law of Constant Proportions is also known as the Law of Constant Composition or Law of Definite Proportions, given by Joseph Proust. 

Statement

He stated the law as 'A chemical compound always contains the same elements combined in the same proportion by mass'.

Example

In a compound such as water, the ratio of the mass of hydrogen and the mass of oxygen is always 1:8, whatever the source of water. 

Similarly, carbon dioxide contains carbon to oxygen in a 3:8(12:32) ratio. In ammonia, nitrogen and hydrogen are present in the ratio of 14:3 by mass. 

Limitations

• Non-stoichiometric compound’s elemental composition will change from one compound to the other. Such compounds follow the Law of Multiple Proportion instead of the Law of Constant Proportion.
• The law does not apply to Isotopes. Isotopes are atoms of the same element which have the same atomic number but different atomic weights. For example, chlorine has two stable isotopes, chlorine-35, and chlorine-37. In the case of HCl or KCl, the masses of the constituents have different proportions because of the isotopic forms of chlorine.

The chapter Atoms and molecules deal with the Atomic theory and different laws of chemical combinations. Problem-based questions will be more from this topic.

1. 5.250g of lead combined with 2.541 g of oxygen to give lead peroxide PbO2. Lead peroxide is synthesized from another method by heating silver nitrate. The percentage of oxygen obtained is 32.61. Illustrate the law of Constant proportion?

• Weight of lead peroxide is 5.250+2.541 = 7.791 g
• 7.791 g of lead peroxide contains 2.541 g of oxygen 
• So 100 g of lead peroxide contains 2.541/7.791*100 = 32.61 g
• The percentage of oxygen obtained by heating lead nitrate is 32.61 by the second method
• Since the percentage composition is the same from both the methods, the data illustrates the Law of Constant Proportion.

2. A sample of calcium carbonate contains 25% of calcium, 11% of carbon, and 43% of oxygen. Applying the Law of Constant Proportion, find the mass of calcium, carbon, and oxygen in 20.0g of calcium carbonate prepared by another method.

• In CaCO3, Ca =28%, C= 15%, O= 57%
• Mass of the second sample of CaCO3 =20.0g
• On the application of the Law of Constant proportion
• Mass of the calcium = 20.0g * 28/100 =5.6g
• Mass of the carbon = 20.0*15/100 =3g
• Mass of the oxygen = 20-(5.6+3) = 11.4 g

3. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

1g of hydrogen reacts with 8 g of oxygen to form water.

For 3g of hydrogen, it requires 3*8= 24 g of oxygen gas. 

Q: Which postulate of Dalton’s Atomic theory explains the Law of Constant Proportion?

Q: Does the Law of Definite Proportions apply to Mixtures and the Elements?

Q: What does the Law of Multiple Proportions say?

Q: What are the Laws of Chemical Combination?

Q: Is the converse of this Law of Constant Proportions true or not?