Question

How many grams of phosphoric acid would be needed to neutralize $100{\text{g}}$of magnesium hydroxide? (The molecular weights of ${H_2}P{O_4} = 98$and $Mg{\left( {OH} \right)_2} = 58.3{\text{ g/mol}}$ respectively)
A.$66.7{\text{g}}$
B.$252{\text{g}}$
C.$112{\text{g}}$
D.$168{\text{g}}$

Answer 1

To solve this question, you must use the concept of equivalents. Equal numbers of equivalents of an acid neutralize equal numbers of equivalents of a base.
Formula Used: $meq = \dfrac{w}{E} = \dfrac{w}{{{M_0}}} \times n$
Where w is weight of compound, E is equivalent weight

Complete step by step answer:
The n factor of a compound is also known as the valence factor and its value depends upon the compound that is being considered. For instance, for an acid, the basicity of the acid is its n factor and for a base, the n factor is its acidity.
In the given question, the reaction between phosphoric acid and magnesium hydroxide can be written as:
$3Mg{\left( {OH} \right)_2} + 2{H_3}P{O_4} \to M{g_3}{\left( {P{O_4}} \right)_2} + 6{H_2}O$
From the reaction, we can conclude that 3 moles of magnesium hydroxide are neutralized by 2 moles of phosphoric acid.
It is given that the mass of magnesium hydroxide used is $100{\text{g}}$
We know that $meq = \dfrac{w}{E} = \dfrac{w}{{{M_0}}} \times n$
The number of milliequivalents of the acid and base must be equal for neutralization reaction. Thus, we can write:
${\left( {\dfrac{w}{E}} \right)_{{H_3}P{O_4}}} = {\left( {\dfrac{w}{E}} \right)_{Mg{{\left( {OH} \right)}_2}}}$
$\dfrac{w}{{98}} \times 3 = \dfrac{{100}}{{58}} \times 2$
Rearranging:
$\Rightarrow w = \dfrac{{100 \times 98 \times 2}}{{58 \times 3}}$
$\therefore w = 112{\text{g}}$

Thus, the correct answer is C.
Note:
Stoichiometry is based upon the very basic laws of chemistry that help to understand it better, namely, the law of conservation of mass, the law of definite proportions (the law of constant composition), the law of reciprocal proportions and the law of multiple proportions . In general, different chemicals combine in definite ratios in chemical reactions. Since matter can neither be created nor destroyed, nor can one element change into the other in a chemical reaction, thus the amount of each element must be the same throughout the entire reaction. For example, the number of atoms of any element in the reactants will be always equal to the number of atoms of that element in the products formed.