Calcium phoshide formed by reacting calcium orthophosphate with `Mg` was hydrolysed by water. The phosphine evolved was burnt in air to yield `P_2O_5`. How many litres of air, containg `21%` by volume of `O_2`, was required to burn `PH_3` and how many grams of magnesium metaphosphate would be obtained if 204 g of Mg was was used for reducing calcium phosphide and the volumes of the gases were measured at STP?

Explanation

To solve the problem step by step, we will break it down into manageable parts based on the chemical reactions and stoichiometry involved.

### Step 1: Write the Chemical Reactions

1. **Formation of Calcium Phosphide**:
\[ 2 \text{Ca}_3(\text{PO}_4)_2 + 8 \text{Mg} \rightarrow \text{Ca}_3\text{P}_2 + 8 \text{MgO} \]
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