Question

(1) If you are trying to freeze a liquid by cooling it, what happens to the temperature? Sketch a

graph of temperature versus time during the cooling.

(2) Define molality. What are its units?

(3) What equation do you use for determining freezing point depression? What do each of the

terms signify?

(4) Pure cyclohexane freezes at 6.6°C. A solution containing 0.37 g of hydroquinone in 25.0 g

cyclohexane was found to freeze at 3.9°C. What is the molar mass of hydroquinone? Cyclohexane

(Kf = 20.0°C•kg/mol)

Answer 1

1)
during freezing temperature is constant with time

Please draw a horizontal line withtemperature on y axis and time on x axis

2)
molality is defined as the number of moles of solute per kilogram of solvent.
The SI unit for molality is mol/kg.

3)
The equation used is:
ΔTf = Kf*m
ΔTf : depression in freezing point , change in temperature
Kf : freezing point depression constant
m : molality in mol/Kg

4)
Given:
delta Tf = (6.6 - 3.9) oC = 2.7 oC
Kf =20.0 oC/m
mass of solvent in Kg = 0.025 Kg
mass of solute = 0.37 g

use:
delta Tf = Kf*mb
2.7 = 20.0 *mb
mb= 0.135 molal

Now use:
molality = mass of solute/(molar mass of solute * mass of solvent in Kg)
0.135 = 0.37/(MM*0.025)
MM = 109.6 g/mol

Answer: 110 g/mol