Question

White phosphorous on reaction with lime water gives calcium salt of an acid (A) along with a gas (X). Which of the following statements is correct?

• A. (A) on heating gives (X) and $O_2$
• B. The bond angle in (X) is less than that of ammonia
• C. (A) is a dibasic acid
• D. (X) is more basic than ammonia

Answer 1

The correct option is B The bond angle in (X) is less than that of ammonia

$2P_4+3Ca(OH{)}_2+6H_{2}O\to 3Ca(H_{2}P{O}_2{)}_2+\underset{\left(X\right)}{2P{H}_3}$

(A) is $H_{3}P{O}_2$ (hypophophorous acid), a monobasic acid.

The bond angle of $P{H}_3$ is less than $N{H}_3$ owing to nitrogen which is more electronegative.
$N{H}_3$ is more basic than $P{H}_3$ as $N{H}_3$ has more electron density owing to the small size of nitrogen and small p orbitals, it has the ability to form stronger bonds due to effective overlap. While in $P{H}_3$, the electron density is diffused throughout the larger p orbital.

$H_{3}P{O}_2$ on heating gives orthophosphoric acid and phosphine (X).
$\underset{\left(A\right)}{2H_{3}P{O}_2}\stackrel{\Delta }{\to }{H}_{3}P{O}_4+\underset{\left(X\right)}{P{H}_3}$