Summary Questions 1. A student determines the molar mass of acetone by the metho

Question

Summary Questions 1. A student determines the molar mass of acetone by the method used in this experiment. The student measured the freezing temperature of pure water to be 0.0 °C and that of a water/acetone m promptly decanted the solution into a beaker and determined the mass of the soluti be 88.0 g. ixture containing 10.0 g acetone to be -4.0 °C. The student a) What is the freezing point depression? b) What is the molality of the solution? ( , water-1.86"C/m) c) How much water was in the decanted solution? (Hint: You need to know the freezing point of pure acetone.) d) How many moles of acetone are present in the decanted solution? (Do not use the molar mass given below in part f). e) Calculate the experimental molar mass of acetone using Eqn. 3. f) Calculate the percent error. (Molar mass of acetone 58.0 g/mol)

Explanation / Answer

Q1

a)

freezing point depression

dTf = Tf-Ti = 0-0.4 = -4°C

b)

molality given

dTf = -Kf*m

m = dTf/Kf = 4/1.86 = 2.15

molal = 2.15

c)

water if

total mass = mass of water + acetone

88 = 10 + X

X = 78 g of water

d)

molality = mol/kg

kg of water = 78*10^-3

mol = 2.15*78*10^-3 = 0.1677 mol

e)

MM of acetone = mass/mol = 10/0.1677 = 59.630 g per mol of acetone

f)

% error = (theoretical - calcualted)/Theorteical * 100% = (58.08 -59.630 )/ 58.08 * 100 = 2.67%

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