Question

Molar Mass Determination by Depression of the Freezing Point

A student determines the freezing point of a solution of 0.630 g of mandelic acid in 20.78 g of tbutanol. He obtains the following temperature-time readings:

Time (min)	Temp (C)	Time (min)	Temp (C)	Time (min)	Temp (C)
0.0	34.9	3.0	22.3	6.0	21.8
0.5	33.3	3.5	21.4	6.5	21.7
1.0	29.7	4.0	22.3	7.0	21.6
1.5	27.1	4.5	22.2	7.5	21.5
2.0	25.2	5.0	22.1	8.0	21.3
2.5	23.6	5.5	22.0

Plot these data using any spreadsheet or plotting program. Note that the first several points fall roughly on a straight line. The last several points fall on a different straight line. Perform linear regressions on these two lines to determine the freezing point of the solution (the temperature at which the lines intersect is the freezing point of the solution). Ignore the rather substantial supercooling that occurred. Attach a copy of the plot to this sheet.

What is the freezing point of the solution?

What is the freezing point depression, ÄTf ? (Tf = 24.5C for t-butanol.)

What is the molality, m, of the mandelic acid in the solution? (Use Equation 1.3, Kf = 8.09 C/m.)

What is the molar mass of mandelic acid? Use Equation 5.4 as rearranged below:

____________C

____________C

____________ m

_________ g/mol

5. The molecular formula of mandelic acid is C8H8O3. Is the result you obtained in Part e consistent with this formula?

Answer 1

a)

What is the freezing point of the solution?

choose the lowest stable point --> 21.3 °C approx

b) What is the freezing point depression, ÄTf ? (Tf = 24.5C for t-butanol.)

dTf = Tpure - Tmix = 24.5-21.3 = 3.2 °C

c) What is the molality, m, of the mandelic acid in the solution? (Use Equation 1.3, Kf = 8.09 C/m.)

m = mol of solute / kg of solvent

via:

dTf = -KF*m

m = dTf/Kf = (3.2)/(8.09) = 0.39555 molal

d) Molar Mass of Mandelic acid

MW = mass of acid / moles of acid

mass used = 0.63 g

kg of solvent = 20.78 g = 20.78*10^-3 kg

molality = mol of solute / kg of solvent

mol of solute = molality * kg of solvent = (0.39555 )(20.78*10^-3) = 0.008219

then

MW = mass/mol = (0.63)/(0.00821)

MW = 76.735 g/mol

Q5.

MW of C8H8O3 --> 8*12+8*1+3*16 = 152 g/mol

ratio = 152/76.735 = 1.98

this is almost twice as the value calculated, there must be an error, specially in the dpression point