Question

The ratio of molecular weight of Hydrogen and Oxygen in water is 1:8. Find the quantity of Oxygen if Hydrogen is $\dfrac{3}{2}$ gms.

Answer 1

Hint: Using molecules weight ratio and proportionality, we will find the weight of Oxygen.

Complete step-by-step answer:
A molecule of water$ = {{\text{H}}_2}{\text{O}}$. It has 2 atoms of hydrogen and 1 atom of oxygen.
Atomic weight of ${\text{H}} = {\text{1u}}$
Atomic weight of ${\text{Q}} = 16{\text{u}}$
Therefore, the ratio of atomic weights $ = \dfrac{{2 \times 1}}{{16u}} = 1:8$ in water.

So for every 1gm of hydrogen, there are 8 gms of oxygen.
Using simple ratios
1 : 8 : : $\dfrac{3}{2}$ : ?
Let unknown weight of oxygen$ = x$ .
Product of means $ = $ product of extremes
$\boxed{x = 8 \times \dfrac{3}{2} = 12{\text{gms}}{\text{.}}}$

Note: It’s important to know that the atom wise ratio of H : O in water is 2 : 1 but the atomic weight wise ratio is 2 : 16 which is 1 : 8.
Also, the term molecular weight has been used interchangeably with atomic weight here in the question, which might confuse some of you just remember, if someone says Hydrogen GAS or oxygen GAS. Then molecular weight $ \ne $ atomic weight. But that’s not been mentioned here so we assume they mean atomic weight.