A 5% solution (by mass) of cane sugar in water has freezing point of 271 K. Calculate the freezing point of 5% glucose in water if freezing point of pure water is 273.15 K.
5% solution means 5g of cane sugar is present in 100g of solution
Freezing point of solution = 271k
Freezing point of pure water = 273.15k
Molar mass of cane sugar(C12H22O11) = 12 × 12 + 1 × 22 + 16 × 11 = 342g
Moles of cane sugar = mass/molar mass = 5/342
⇒ n = 0.0146mol
Molality of solution = moles of solute/mass of solvent(in kg)
⇒ M = 0.0146/0.095
⇒ Molality = 0.154M
Depression in freezing point = ΔTf = 273.15-271 = 2.15k
Applying the formula: ΔTf = Kf × M
Where
ΔTf = depression in freezing point
Kf = molal depression constant
M = molality of solution
⇒ Kf = 2.15/0.154
⇒ Kf = 13.96k kg mol-1
Second condition: mass of glucose = 5g
Molar mass of glucose(C6H12O6) = 12 × 6 + 1 × 12 + 16 × 6 = 180g
Moles of glucose = mass/molar mass
⇒ n = 5/180moles
⇒ n = 0.0278mol
Molality of solution = moles of solute/mass of solvent(in kg)
⇒ molality = 0.0278/0.095
⇒ M = 0.2926M
Applying the formula: ΔTf = Kf × M
Where
ΔTf = depression in freezing point
Kf = molal depression constant
M = molality of solution
⇒ ΔTf = 13.96 × 0.2926
⇒ ΔTf = 4.08k
Freezing point of solution = 273.15 + 4.08 = 277.234k