Question

What is the freezing point of a solution containing $8.1\text{g HBr}$ in $100gm$. water assuming the acid to be $90\%$ ionised:

[$K_f$ for water $=1.86K{\text{molarity}}^{-1}$]

• A. ${0.85}^{\circ }C$
• B. $-{3.53}^{\circ }C$
• C. $0^{\circ }C$
• D. $-{0.35}^{\circ }C$

Answer 1

Answer: (B)

$-{3.53}^{\circ }C$

Concentration of solute $=8.1$ gm and concentration of solvent $=100$ gms

$\alpha =0.9$

We know,

$\Delta T_f=i{K}_{f}m$

$=1+(n-1)\alpha \times 1.86\times \frac{8.1}{80}\times \frac{100}{100}$

$\Delta T_f=1.91.861=3.53{}^{0}C$

Therefore, freezing point of the solution $=0-3.53=-3.53$ degree Celsius

Hence, the correct option is $B$