Question

1.06 g of Na$_2$CO$_3$ (molar mass = 106 g mol$^{-1}$) is dissolved in 500 g water. What is its molality?

• A. 0.2 m
• B. 0.02 m
• C. 2 m
• D. 0.04 m

Hint:

Always convert solvent mass to kg when calculating molality.

Answer 1

The Correct Option is B

Molality $(m) = \dfrac{\text{moles of solute}}{\text{mass of solvent in kg}}$
Moles of Na$_2$CO$_3$ = $\dfrac{1.06}{106} = 0.01$ mol
Mass of water = 500 g = 0.5 kg
$m = \dfrac{0.01}{0.5} = 0.02$ mol/kg