Question

Pure nitrobenzene freezes at 5.67 C. When 1.0g of ethanol (C2H6O) is mixed with 20.0g nitrobenzene, the freeze point drops to –0.53 C. What is the freezing-point depression constant (Kf) of nitrobenzene?

Answer 1

Answer: The freezing-point depression constant (Kf) of nitrobenzene is $5.7^0C/m$

Explanation:

Depression in freezing point is given by:

$\Delta T_f=i\times K_f\times m$

$\Delta T_f=T_f^0-T_f=(5.67-(-0.53)^0C=6.2^0C$ = Depression in freezing point

i= vant hoff factor = 1 (for non electrolyte nitrobenzene)

$K_f$ = freezing point constant = ?

m= molality

$\Delta T_f=i\times K_f\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

Weight of solvent = 20 g = 0.02 kg

mass of solute (ethanol) = 1.0 g

Molar mass of ethanol = 46 g/mol

$6.2=1\times K_f\times \frac{1.0g}{46g/mol\times 0.02kg}$

$K_f=5.7^0C/m$

Thus freezing-point depression constant (Kf) of nitrobenzene is $5.7^0C/m$