Question

How many grams of phosphoric acid would be needed to neutralise $100$ g of magnesium hydroxide? (The molecular weight of $H_{2}P{O}_4=98$ g/mol and $Mg(OH{)}_2=58.3$ g/mol)

• A. $66.7g$
• B. $252g$
• C. $112g$
• D. $168g$

Answer 1

Answer: (C)

$112g$

$3Mg{\left(OH\right)}_2+2H_{3}P{O}_4\to M{g}_3{\left(P{O}_4\right)}_2+6H_{2}O$

$3\times 58.3g\left(Mg{\left(OH\right)}_2\right)$ reacts with $\to 2\times 98g\left(H_{3}P{O}_4\right)$

$100g$ reacts with $\begin{array}{c}\to \frac{100\times 2\times 98}{3\times 58.3}\\ =112g\end{array}$