Question

How many grams of phosphoric acid would be needed to neutralise 100 g of magnesium hydroxide? (The molecular weight of ${H_3}P{O_4}$ = 98 g/mol and $Mg{(OH)_2}$= 58.3 g/mol)
A) 66.7 g
B) 252 g
C) 112 g
D) 168 g

Answer 1

Hint: Use mole concept to reach the answer of this question. You first need to write the neutralisation balanced chemical reaction between magnesium hydroxide and phosphoric acid. Then, calculate the number of moles of magnesium hydroxide and phosphoric acid and then, relate them.

Complete step by step answer:
In the question, we are asked that in the neutralisation reaction of magnesium hydroxide and phosphoric acid, how many grams of phosphoric acid would be required to neutralise given 100 g of magnesium hydroxide.
The neutralisation reaction between phosphoric acid ($Mg{(OH)_2}$) and magnesium hydroxide ($MgP{O_4}$) is as given below:
${H_2}P{O_4} + Mg{(OH)_2} \to MgP{O_4} + 2{H_2}O$

From the above neutralization reaction, you can see that 1 mol of ${H_2}P{O_4}$ neutralises 1 mole of $Mg{(OH)_2}$.
Molar mass of ${H_2}P{O_4}$= 98 g
Molar mass of $Mg{(OH)_2}$= 58.3 g
Given mass of $Mg{(OH)_2}$= 100 g

Now, calculating moles of involved species in the reaction:
Formula for calculating moles = $\dfrac{{{\text{Given mass}}}}{{{\text{Molar mass}}}}$
Number of moles of $Mg{(OH)_2}$= $\dfrac{{100}}{{58.3}} = 1.72{\text{ moles}}$
Let, given mass of ${H_2}P{O_4}$ = $x$ g
Therefore, number of moles of ${H_2}P{O_4}$= $\dfrac{x}{{98}}$
Since, it is clear from the neutralisation reaction that the number of moles of ${H_2}P{O_4}$ and $Mg{(OH)_2}$ used for the reaction are equal i.e., equals to one.
Therefore, we can equate calculated moles of ${H_2}P{O_4}$ and $Mg{(OH)_2}$ as:
$\dfrac{x}{{98}}$ = 1.72
Therefore, $x = 98 \times 1.72 = 168.56{\text{ }}g \simeq 168{\text{ }}g$
Hence, 168 grams of phosphoric acid would be needed to neutralise 100 g of magnesium hydroxide.
So, the correct answer is “Option D”.

Note: A neutralization is a chemical reaction in which an acid reacts with a base to form salt and water as products. Basically, in a neutralization reaction there is a combination of ${H^ + }$ ions from the acid and $O{H^ - }$ ions from the base. In the neutralization reaction of ${H_2}P{O_4}$ and $Mg{(OH)_2}$, ${H_2}P{O_4}$ is acid, $Mg{(OH)_2}$ is the base and the salt produced is $MgP{O_4}$, magnesium phosphate.