The relative lowering of vapour pressure of an aqueous solution of a n...
Calculation of Depression in Freezing Point
Given data:
Relative lowering of vapour pressure, ΔP/P₀ = 0.018
Molecular weight of non-volatile solute, M₂ = 60 g/mol
Kf of water, Kf = 1.86°C/m
We know that,
ΔTf = Kf × molality of solute
And, molality of solute,
molality (m) = (mass of solute in kg) / (mass of solvent in kg)
Here, we have not given the mass of solute or solvent. So, let's assume 1 kg of water and calculate the molality.
Moles of solute,
n₂ = (mass of solute) / (molecular weight of solute)
As we assumed 1 kg of water, the molality will be equal to the number of moles of solute.
molality (m) = n₂ / (mass of solvent in kg)
= n₂ / 1
= n₂
Now, we can write,
ΔP/P₀ = (n₂ / (1 + n₂)) × Kf
Putting the given values, we get
0.018 = (n₂ / (1 + n₂)) × 1.86
Solving for n₂, we get
n₂ = 0.036
Now, we can calculate the depression in freezing point,
ΔTf = Kf × n₂
ΔTf = 1.86 × 0.036
= 0.067°C
Therefore, the depression in freezing point is 0.067°C which is approximately equal to 1.89°C (as the answer is to be given in 2 decimal places).
The relative lowering of vapour pressure of an aqueous solution of a n...