Question

The freezing point of 1 molal NaCl solution assuming NaCl to be 100% dissociated in water is:

• A. $-{1.86}^{o}C$
• B. $-{3.72}^{o}C$
• C. $+{1.86}^{o}C$
• D. $+{3.72}^{o}C$

Answer 1

The correct option is B $-{3.72}^{o}C$

The $i$ value for $NaCl$ solution assuming $NaCl$ to be 100% dissociated is $2$ .

$k_f=$ molal freezing point constant or cryoscopic constant $=1.86$ K·kg/mol

$\therefore$ Depression in freezing point $=i\times$ $K_f$ $\times$ molality

$=2\times 1.86\times 1$

$=3.72$

$T_f=0-\Delta T_f=0-3.72=-3.72{}^{0}C$

Hence, the correct option is $B$