Question

Which one of the following sets of compound correctly illustrates the law of reciprocal proportions?

• A. $P_2{O}_3,P{H}_3,H_{2}O$
• B. $P_2{O}_5,P{H}_3,H_{2}O$
• C. $N_2{O}_5,N{H}_3,H_{2}O$
• D. $N_{2}O,N{H}_3,H_{2}O$
• E. $N{O}_2,N{H}_3,H_{2}O$

Answer 1

The correct option is B $P_2{O}_5,P{H}_3,H_{2}O$

In $P{H}_3$, the ratio by weight of $P:H=31:3$

For $H_{2}O,O:H=16:2=8:1$

Keeping the weight of $H(=1)$ fixed, $P:O=\frac{31}{3}:\frac{8}{1}=31:24⟶\left(1\right)$

In $P_2{O}_5,P:O$ is $(2\times 31):(5\times 16)$

$=62:80$ or $31:40⟶\left(2\right)$

Keeping the weight of $P(=31)$ fixed in equation (1) & (2), the ratio of oxygen is $24:40$ or $3:5$ which is a simple ratio.