Question

An aqueous solution of sodium chloride is marked $10\%$ (w/w) on the bottle. The density of the solution is 1.071 gml.${}^{-1}$. What is its molality and molarity respectively?

• A. 1.73 m and 1.56 M
• B. 1.56 m and 1.73M
• C. 1.90 m and 1.83 M
• D. 1.83 m and 1.90 M

Answer 1

The correct option is C 1.90 m and 1.83 M

$100g$ solution contains $10g$ $NaCl$ $w=10g$ , $m_{NaCl}=58.5$, Volume of solution $=\frac{100}{1.071\times 1000}litre$ $w_{H_{2}O}=100-10=90$ g
Molarity =$\frac{\text{Wt. of solute}}{\text{mol. wt. of solute}\times V_{inL}}=\frac{10\times 1.071\times 1000}{58.5\times 100}=1.83M$
Molarity $=\frac{\text{Wt. of solute}}{\text{mol. wt. of solute}\times \text{weight of solvent in Kg}}$
$\frac{10\times 1000}{58\times 90}=1.90m$