Freezing Point Cyclohexane (Solvent) I. Mass of test tube and cyclohexane (grams

ID: 481065 • Letter: F

Question

Freezing Point Cyclohexane (Solvent) I. Mass of test tube and cyclohexane (grams) _________ Mass of test tube (grams) _________ Mass of cyclohexane (grams) ________ Freezing point, from cooling curve (degree C)__________ Instructor's approval of graph _____________ Freezing Point of Cyclohexane and known Solute Trial 1 Trial 2 Unknown Solute ID __________ Mass of test tube and cyclohexane (grams) ______ _______. Mass of cyclohexane (grams) _______ ________ Mass of solute, total (grams) _______ ___________ Freezing point, from cooling curve (degree C) ________ _________ Calculations k _f for cyclohexane 20.0 degree C kg/mol Freezing point change, delta Tr (degree C) _________ _________ Mass of solute in solution, total (grams) _________ _________ Mass of cyclohexane in solution (kg) _________ _________ Moles of solute (mol) _________ _________ Molar mass of solute (g/mol) _________ _________ Average molar mass of solute _________

Explanation / Answer

Calculations

1. kf for cyclohexane

20.0 C.kg/mol

2. Freezing point change, Tf (C)

0.7

3. Mass of solute in solution, total (grams)

0.151

0.14

4. Mass of cyclohexane in solution (kg)

9.011*10-3 (see calculation 1 below)

7.353*10-3

Moles of solute (mol)

3.15385*10-4 (see calculation 2 below)

2.57355*10-4

Molar mass of solute (g/mol)

478.78 (see calculation 3 below)

543.99

Average molar mass of solute (g/mol)

(478.78 + 543.99)/2 = 511.385

Calculation 1: Mass of cyclohexane (g) = (9.011 g)*(1 kg/1000 g) = 9.011*10-3 kg

Calculation 2: Use the law of freezing point depression

Tf = kf. m where m = molality of the solution.

Plug in values.

0.7C = (20.0C.kg/mol)*m

===> m = 0.7/20.0 mol/kg = 0.035 mol/kg

We know that molality of a solution = moles of solute/kg of solvent.

We have

0.035 mol/kg = moles of solute/(9.011*10-3 kg)

===> moles of solute = (0.035)*(9.011*10-3) mole = 3.15385*10-4

Calculation 3: Moles of solute = mass of solute in grams/molar mass of solute in grams