A student measures the mass of a beaker with an empty test tube in it, and finds

Question

A student measures the mass of a beaker with an empty test tube in it, and finds that the mass is 156.852 grams. They then add approximately 15 mL of pure cyclohexane to the test tube and reweigh the beaker and test tube with cyclohexane in it. The beaker, test tube and cyclohexane weighs 171.206 grams. Then they measure the freezing point of the pure cyclohexane and find that it is 6.60 degree C. The student then adds 0.300 grams of an unknown hydrocarbon to the cyclohexane in the test tube and stirs it until it has dissolved. Finally, the freezing point of the cyclohexane and hydrocarbon solution was measured and found to be 1.25 degree C. The freezing point depression constant for cyclohexane is 20.2 degree C/m. 1. What is the change in temperature of the freezing point? Delta T of freezing point: _____ 2. What is the molality of the unknown hydrocarbon in the solution? Molality of hydrocarbon: _____ 3. What is the mass of cyclohexane in the test tube in kilograms? Mass of cyclohexane: _____ 4. How many moles of the unknown hydrocarbon are present in the solution? Moles of hydrocarbon: _____ 5. What is the molar mass of the unknown hydrocarbon? Molar Mass of hydrocarbon: _____

Explanation / Answer

(1)

Change in freezing point = 6.60 - 1.25 = 5.35 0C

(2)

We know that,

depression freezing point = molal depression constant * molality

5.35 = 20.2 * molality

molality = 0.2648 m

(3)

Mass of solvent, cyclohexane, W = 171.206 - 156.852 = 14.354 g. = 0.014354 kg.

(4)

molality = moles of solute / mass of solvent in kg

0.2648 = moles of solute / 0.014534

Moles of solute = 0.003849 mol

(5)

Mass of solute, w = 0.300 g.

Molar mass = mass / moles = 0.300 / 0.003849 = 77.94 g/mol

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