Cyclohexane has a freezing point of 6.50 ∘C and a Kf of 20.0 ∘C/m. What is...

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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 dissolving0.771 g of biphenyl (C12H10) in 25.0 g of cyclohexane?

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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


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