Question

Choose the most correct option.
A sample of pure water, whatever the source always contains ___________ by mass of oxygen and 11.1 % by mass of hydrogen.
a) 88.8
b) 18
c) 80
d) 16

Answer 1

Hint: Recall the law of constant or definite proportions. Chemical formula of water is ${H_2}O$. Also, you must know the formula for calculating the percent composition of an element in a compound. Calculate the mass percent of oxygen in water to get the answer.

Complete answer:
According to the Law of definite proportion, a given compound always contains exactly the same proportion of elements by weight. It simply means that irrespective of the source, a given compound always contains the same elements in the same proportion or in fixed ratio.
Now, we are given a sample of pure water and 11.1 % by mass of hydrogen, and are asked about the percent by mass of oxygen. Chemical formula of water is ${H_2}O$. Water contains two hydrogen atoms and one oxygen atom.
Molar mass of water, ${H_2}O$= $2 \times {\text{atomic mass of hydrogen + }}1 \times {\text{atomic mass of oxygen}}$
Therefore, molar mass of ${H_2}O$= $2 \times 1 + 16 = 18g$.
Mass of oxygen in ${H_2}O$: 16 g
Mass of hydrogen in ${H_2}O$: 2 g
Formula for calculating percent composition of any element in a given compound is:
Mass percent of an element = $\dfrac{{{\text{Mass of that element in the compound }}}}{{{\text{Molar mass of compound}}}} \times 100$
So, mass % of hydrogen in ${H_2}O$ = $\dfrac{{{\text{mass of hydrogen in }}{{\text{H}}_{\text{2}}}{\text{O}}}}{{{\text{Molar mass of }}{{\text{H}}_{\text{2}}}{\text{O}}}} \times 100 \Rightarrow \dfrac{2}{{18}} \times 100 = 11.1\% $. (We have calculated this, but it is also given in the question).
Mass % of oxygen in ${H_2}O$ = $\dfrac{{{\text{Mass of oxygen in }}{{\text{H}}_{\text{2}}}{\text{O}}}}{{{\text{Molar mass of }}{{\text{H}}_{\text{2}}}{\text{O}}}} \times 100 \Rightarrow \dfrac{{16}}{{18}} \times 100 = 88.8\% $.
Therefore, a sample of pure water, whatever the source, always contains 88.8% by mass of oxygen and 11.1 % by mass of hydrogen.

Thus, option (a) is correct.

Note:
Whatever the source of water, it always contains hydrogen and oxygen combined together in the fixed ratio by mass i.e. 2:16 or 1:8. For example, if you have taken one sample of water from sea and another sample from river, both samples have the same composition of hydrogen and oxygen.