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`

Step-by-step explanation:

Depression in freezing point is given by:

`\Delta T_f=i* K_f* 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* K_f* \frac{\text{mass of solute}}{\text{molar mass of solute}* \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* K_f* (1.0g)/(46g/mol* 0.02kg)`

`K_f=5.7^0C/m`

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