Question

Calculate the mass of a compound (molar mass = 256 g mol⁻¹) to be dissolved in 75 g of benzene to lower its freezing point by 0.48 K (K_f = 5.12 K kg mol⁻¹).

Answer 1

\[Given: \]
\[\Delta T_f = 0 . 48 K\]
\[ K_f = 5 . 12 \text{K kg}\ \text{mol}^{- 1} \]
\[ w_1 = 75 g\]
\[ w_2 = ?\]
\[ M_2 = 256 g\ \text{mol}^{- 1} \]
\[\text{Using}\]
\[\Delta T_f = \frac{K_f \times w_2 \times 1000}{M_2 \times w_1}\]
\[ \Rightarrow w_2 = \frac{∆ T_f \times M_2 \times w_1}{K_f \times 1000} = \frac{0 . 48 \times 256 \times 75}{5 . 12 \times 1000} = 1 . 8 g\]