Question

NaCl is added to 1 litre water to such an extent that $\Delta T_f/K_f$ becomes to $\frac{1}{500}$, the wt. of NaCl added is

• A. $5.85g$
• B. $0.585g$
• C. $0.0585g$
• D. None of these

Answer 1

Answer: (C)

For $NaCl,i=2$
$\therefore \frac{\Delta T_f}{K_f}=2\times C_m$
$\therefore \Delta T_f=K_f\times i\times C_m$
or, $\frac{1}{500}=2\times$ molality
or molality $=\frac{1}{1000}$
i.e., $\frac{1}{1000}$ moles $k{g}^{-1}$ water
i.e., $\frac{1}{1000}$ moles per litre water
$=\frac{1}{1000}\times 58.5gNacl$
$=0.0585gNacl$