Question

$25 \,g$ of a solute of molar mass $ 250\,g\,mol^{-1} $ is dissolved in $10\, mL$ of water to obtain a solution whose density is $ 1.25\,g(mL)^{-1}. $ The molarity and molality of the solution are respectively

• A. $0.75\, M$ and $1\, m$
• B. $0.8\, M$ and $1\, m$
• C. $1 \,M$ and $0.8\, m$
• D. $1\, M$ and $0.75\, m$

Answer 1

Answer: (C)

Molarity $=\frac{\text { weight of solute } \times 1000}{\text { mol.wt.of solute } \times \text { volume of solution }}$
$=\frac{25 \times 1000}{250 \times 100}$
$\left[\because\right.$ density of water $\left.=1 g( mL )^{-1}\right]$
$=1 M$ Molality $=\frac{\text { weight of solute } \times 1000}{\text { mol.wt.of solute } \times \text { w.of solvent }}$
$=\frac{25 \times 1000}{250 \times 100}=1 m .$