Question

A solution of $H_{3}P{O}_4$ is 98% by mass. Calculate the molarity and normality if the density of the solution is 1.8 g/mL.

• A. 18 M, 18 N
• B. 18 M, 54 N
• C. 6 M, 18 N
• D. 54 M, 18 N

Answer 1

The correct option is B 18 M, 54 N

Let the mass of the solution be 100 g.
98% by mass $H_{3}P{O}_4$ means 98 g of $H_{3}P{O}_4$ is present in 100 g of the solution.
Number of moles = $\frac{\text{given mass}}{\text{molar mass}}=\frac{98}{98}=1\text{mol}$

Density = $\frac{\text{mass}}{\text{volume}}$
$\Rightarrow$ volume of the solution = $\frac{100}{1.8}$ mL

$\text{Molarity}=\frac{\text{moles of solute}}{\text{total volume of solution (in mL)}}\times 1000=\frac{1}{\frac{100}{1.8}}\times 1000$
$\text{Molarity}=18M$
Normality = Molarity $\times$ n-factor
(n-factor for $H_{3}P{O}_4$ is 3)
Normality = 54 N