Cyclohexane has a freezing point of 6.50 C and a K f of 20.0 C/ m . What is the

Question

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?

Express the temperature numerically in degrees Celsius.

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Part B

Paradichlorobenzene, C6H4Cl2, is a component of mothballs. A solution of 2.00 g in 22.5 g of cyclohexane boils at 82.39 C. The boiling point of pure cyclohexane is 80.70 C. Calculate Kb for cyclohexane.

Express the constant numerically in degrees Celsius per molal.

Explanation / Answer

Tf normal = 6.5; Kf = 20

dTf = Kf*m

m = mol S / kg solvent = (0.771/154.21) / (25*10^-3) = 0.19998

dTf = 20*0.19998 = 3.9996

Tf new = 6.50-3.9996 = 2.5004 °C

Tf = 2.5°C

B)

m = 2 g --> mol = mass/MW = 2/147.01 = 0.0136 mol of C6H4Cl2

kg solvent = 22.5*10^-3 kg

m = mol S / kg solvent = 0.0136 /(22.5*10^-3)=0.60444 molal

dTb = Kb*m

dTb = (82.39-80.70) = 1.69

Kb = dTb/m = 1.69/0.60444 = 2.795 °C/m

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