Question

A mixture of hydrogen and oxygen $\left(45ml\right)$ is sparked to form liquid water. The component not in excess reacts completely and $15ml$ is left over. (All measurements are made at the same temperature and pressure). The composition by volume in the original mixture of $H_2:O_2$ is _____.

• A. 4:5
• B. 7:2
• C. 2:1
• D. either 4:5 or 7:2

Answer 1

Answer: (D)

either 4:5 or 7:2

Reaction for formation of water is

${2H}_2+O_2\to {2H}_{2}O$

Let us assume the excess is $H_2$.

15 mL of $H_2$ is left.

In remaining 30 mL,

$H_2$ = 20 mL & $O_2$ = 10 mL

So, original mix has 35 mL of $H_2$ and 10 mL of $O_2$.

So, the ratio of $H_2$ & $O_2$ = 35 : 10 = 7 : 2

Now, assume the excess is $O_2$.

15 mL of $O_2$ is left.

In remaining 30 mL:

$H_2$ = 20 mL & $O_2$ = 10 mL.

So, original mix has 20 mL of $H_2$ and 25 mL of $O_2$.

So, the ratio of $H_2$ & $O_2$ = 20 : 25 = 4 : 5

Hence, option D is correct.