Answer : The formula of the gas produced is,
(phosphine gas)
Explanation :
According to the question, when sodium phosphide is treated with water then it react to give phosphine gas and sodium hydroxide.
The balanced chemical reaction will be:

By Stoichiometry of the reaction we can say that:
1 mole of sodium phosphide reacts with 3 moles of water to give 1 mole of phosphine gas and 3 moles of sodium hydroxide.
Thus, the formula of the gas produced is,
(phosphine gas)
a. 2Na Al 03
Equation must have the same number of each element on both sides of the equation.
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