Question

Law of constant proportion states that:
A. A chemical compound always contains exactly the same proportion of elements by mass.
B. Mass can neither be created nor destroyed in a chemical reaction.
C. When two elements combine with each other to form two or more than two compounds, the mass of one of elements which combines with the fixed mass of the other, bears a simple whole number ratio.
D. Total mass of products is always equal to the total mass of reactants.

Answer 1

Hint: The Law of constant proportion is related to the mass of elements and their proportions in a chemical compound.

Complete answer:
The law of constant proportion states that a chemical compound contains the same proportion of elements by mass. This means that the elements that make up the chemical compounds have masses of fixed ratio in that chemical compound. In a more detailed manner, this law implies that if we take a sample of a compound which is free from all impurities, that sample will contain the elements in the same ratio by mass, no matter what the source is. This law is sometimes known as the law of definite proportions or Proust’s Law.
Let us consider the example of a water molecule. The water molecule is composed of two elements – hydrogen and oxygen. There are two hydrogen atoms and one oxygen atom in a molecule of water. Its formula is ${ H }_{ 2 }O$.
Now, the atomic mass of hydrogen is 1 amu and oxygen is 16 amu. There are 2 hydrogens, so the total atomic mass of ${ H }_{ 2 }$ is $1\times2=2 amu$. Now, the total mass of water is 16+2=18 amu and the ratio of hydrogen to oxygen by mass is 2:16 = 1:8. According to this law, this ratio of the elements by mass of water molecules will always remain constant.
Therefore, the total mass of water will be 8+1=9 units out of which 8 units are oxygen and 1 unit is hydrogen. In any given mass of water, $\dfrac { 8 }{ 9 }$ part of it will be oxygen and $\dfrac { 1 }{ 9 }$ part of it will be hydrogen. So, if we take 1 gm of water, it will always contain $\dfrac { 1 }{ 9 }gm$ of hydrogen and $\dfrac { 8 }{ 9 }gm$ of oxygen.
This ratio will remain the same even if we take samples of water from different sources such as seawater, lakes, rivers ponds, etc.

Note: Don’t confuse between the law of constant proportions and the law of multiple proportions. The law of multiple proportions is defined in option C and this law is more appropriate for the elements which form different compounds between them.