Question

Phosphorus (at. mass 31) combines with hydrogen (at. mass 1) in the mass ratio of $3.1: 0.3$. In water, hydrogen and oxygen (at. mass 16) combine in the mass ratio $0.2: 1.6$.

(i) From the above data, calculate the mass ratio of phosphorus and oxygen in the oxide of phosphorus

(ii) What is the name of the law that is applied in calculating this ratio?

(iii) What is the valency of phosphorus in an oxide where the mass ratio of phosphorus to oxygen is $3.1: 4$?

Answer 1

(i) According to the given data, $0.3 g H _{2}$ combincs with phosphorus $=3.1 g$

$1 g H _{2}$ combines with phosphorus $=\frac{31}{3} g$

In water, $0.2 g H _{2}$ combines with oxygen $=1.6 g$

$1 g H _{2}$ combines with oxygen $=\frac{1.6}{0.2}=8 gm$

Ratio between the weights of phosphorus and oxygen which combine with fixed weight of $H _{2}(1 g )$ is $\frac{31}{3}: 8$ or $31: 24$

Therefore, the ratio between the weights of phosphorus and oxygen when they directly combine to form an oxide of phosphorus should be $31: 24$ or some simple multiple of it.

(ii) The law used to calculate the mass ratio fo phosphorus and oxygen is known as law of reciprocal or cquivalent proportions.

(iii) Oxide in which mass ratio of phosphorus to oxygen is $3.1: 4$ is $P_{2} O_{5}$ and valency of phosphorus in it is $5 .$