Question

$40$g MgO dissolves in $H_{2}O$ to form $200$ ml of solution, if density of solution is $1.5$g/ml. Calculate the molality of the solution.

Answer 1

MgO is solute & $H_{2}O$ is solvent,

Weight of solute $=40g$

Density of solution $=1.5g/mL$

Volume of solution $=200ml$

$\therefore$ mass of solution (weight) $=$ Density $\times$ volume

$=1.5\times 200=300g$

Weight of solvent $=$ Weight of solution $-$ weight of solute

$=300-40$

$=260g$

Moles of solute $=\frac{\text{weight of solute}}{\text{Molecular weight of solute}}=\frac{40}{40}=1mole$

molality (m) $=\frac{\text{No of moles of solute}}{\text{Weight of solvent in kg}}=\frac{1}{0.260}=3.84mol/kg=m$