Question

Which of the following sets of compounds 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. $N_{2}O,N{H}_3,H_{2}O$

Answer 1

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

The ratio of a number of hydrogen and oxygen combining with one $P$ is $3:1.5$ i.e., $2:1$. It is same as in water $(2:1)$.

Law of Reciprocal proportions:-

When two different elements combine with the same mass of a 3rd element, the ratio of masses in which they do so must be the same or multiple of the mass ratio in which they combine with each other.

A.The ratio of phosphorous combined with oxygen in$P_2{O}_3$ is

P:O =62:48 = 31 : 24

B.The ratio of Phosphorous combining with hydrogen in $P{H}_3$ is

P : H = 31 : 3

C. The ratio of hydrogen combined with oxygen in $H_{2}O$ is

H:O= 2:16=1:8 ⟶1

Now, the ratio of hydrogen & oxygen combined with P

3:24=1:8 ⟶2

From 1 & 2, 1:8 is the same in both cases

∴ The Law of reciprocal proportions is illustrated correctly.