In: Chemistry
Cyclohexane has a freezing point of 6.50 ∘C and a Kf of 20.0 ∘C/m. What is the freezing point of a solution made by dissolving 0.771 g of biphenyl (C12H10) in 25.0 g of cyclohexane?
Answer – We are given, Cyclohexane freezing point = 6.50∘C , Kf = 20.0oC/m
Mass of biphenyl = 0.771 g , mass of cyclohexane = 25.0 g
First we need to calculate the moles of biphenyl
Moles of biphenyl = 0.771 g / 154.211 g.mol-1
= 0.005 moles
So molality of biphenyl = 0.005 moles / 0.025 kg = 0.200 m
We know the formula for freezing point depression
ΔTf = Kf*m
= 20.0oC/m * 0.200 m
= 4.00oC
We know the
Freezing point = Freezing point of pure solvent – ΔTf
= 6.50oC – 4.00oC
= 2.4oC