Question

How many moles of methanol must be added to 4.50 kg of water to lower its freezing point to -10.0 ∘c? for each mole of solute, the freezing point of 1 kg of water is lowered 1.86 ∘
c.

Answer 1

We use the concept on colligative properties to calculate the number of moles in the system above. We calculate as follows:

ΔT = -kf(m)

where ΔT is the change in freezing point, kf is the freezing point depression and m is the molality of the solution.

-10 = -1.86(m)
m = 5.37 mol/kg

number of moles = 5.37 (4.50) = 24.19 moles of methanol