Question

One mole of calcium phosphate on reaction with excess water gives:
A. 1 mol of phosphine
B. 2 mol of phosphoric acid
C. 2 mol of phosphine
D. 1 mole of phosphorus penta-oxide.

Answer 1

The molecular formula of calcium phosphine is $\text{C}{{\text{a}}_{3}}{{\text{P}}_{2}}$ when it reacts with water in the excess amount it yields calcium hydroxide ($\text{Ca}{{\left( \text{OH} \right)}_{2}}$) and phosphine gas ($\text{P}{{\text{H}}_{3}}$). The gas formed is flammable and poisonous.

Complete step by step answer:
-It is given that excess water is present, it is necessary because when the phosphine is formed it may catch and can be harmful.
-So, to avoid this condition excess of water is required.
-Firstly, we will write the skeletal equation of the reaction between calcium phosphate and water:
$\text{C}{{\text{a}}_{3}}{{\text{P}}_{2}}\text{ }+\text{ }{{\text{H}}_{2}}\text{O }\to \text{ Ca}{{\left( \text{OH} \right)}_{2}}\text{ + P}{{\text{H}}_{3}}$

-Now, we have to balance the equation:
-Here, six molecules of water are formed because a total of 6 molecules of oxygen are present at the product side.
-3 molecules of calcium hydroxide are formed because a total of 3 molecules of calcium are present at the reactant side.
-2 molecules of phosphine are formed because a total of 2 molecules of phosphorus are present at the reactant side.
-So, the equation will be balanced in this way.
$\text{C}{{\text{a}}_{3}}{{\text{P}}_{2}}\text{ + 6}{{\text{H}}_{2}}\text{O }\to \text{ Ca(OH}{{\text{)}}_{2}}\ \text{+ 2P}{{\text{H}}_{3}}$

-Now, we can see that two moles of phosphine are formed at the end of the reaction.
So, the correct answer is “Option C”.

Note: The given reaction is helpful in the synthesis of phosphine, which is used in the semiconductors, also acts as a polymerization initiator, etc. And calcium hydroxide, which is used in the construction, sewage treatment, whitewashing, etc.