Question

A solution of $H_{3}P{O}_4(d=1.8\text{g/ml})$ has a molarity of $18\text{M}$. Hence, mass percentage and molality are:

• A. 18%, 32.4 molal
• B. 98%, 32.4 molal
• C. 98%, 500 molal
• D. 98%, 18 molal

Answer 1

The correct option is C 98%, 500 molal

$18\text{M}{H}_{3}P{O}_4=18\text{mol}{H}_{3}P{O}_4\text{in}1000\text{ml solution}$

Given, density of the solution $d=1.8\text{g/ml}$

So mass of $1000\text{ml solution}1.8\times 1000=1800\text{g}$
Mass of $18\text{mol}{H}_{3}P{O}_4=18\times 98=1764\text{g}$

So mass percentage of $H_{3}P{O}_4=\frac{1764}{1800}\times 100=98\%$

$\therefore \text{Molality}=\frac{18}{36}\times 1000=500\text{m}$