Experiment 13 Advance Study Assignment: Determination of Molar Mass by Depressio

Question

Experiment 13 Advance Study Assignment: Determination of Molar Mass by Depression of the Freezing Point 1. A student determines the freezing point of a solution of 0.63 g of mandelic acid in 20.78 g of ter- tiary butyl alcohol. He obtains the following temperature-time readings Time (min) 00.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Temp (°C) 34.9 33.3 29.7 27.1 252 23.6 22.3 21.4 22.3 Time (min)45 5.0 5.5 60 6.57.0 75 8.0 Temp, (oC) 22.2 22.1 22.0 21.8 21.7 21.6 21.5 21.3 Plot these data on the graph paper provided. Note that the first several points fall roughly on a. a straight line. The last several points fall on a better straight line. Draw in those two straight lines as best you can. The point at which the lines intersect is the freezing point of the solu- tion. Ignore the rather substantial supercooling that occurred b. What is the freezing point of the solution? c. What is the freezing point depression, T-T, or AT? Take T? to be 24.5 C for TBA. °C d. What is the molality m of the mandelic acid in the solution? (Use Eq. 1: 8.09.) ·2224m e. What is the molar mass of mandelic acid? Use Equation 2 as modified below MM = no. g solute 125.2 no. kg solvent m f. The molecular formula of mandelic acid is C.H,CH (OH)-COOH. Is the result you obsained in Part e consistent with this formula? Any comments? n part e is les than the acta 12x8 The molar mass IS2 molar vmaJ of mande lic Ocid. Ollo 3 95

Explanation / Answer

Solution:

b) Freezing point = 64.4 C

c) freezing point depression = Tf'- Tf = 68.2 - 64.4 = 3.8 C

d)

we have Kf = 4.89

so, freezing point depression = Tf'- Tf = Kf*m

m = 3.8/4.89 = 0.77709

molality of Naphthalene = 0.77709 m

e)

MM = (no.g solute / no.Kg solvent ) * (Kf/ Tf'- Tf) = (2.07 / 0.021) * (4.89 / 3.8) = 126.84586

so molar mass of Naphthalene = 126.84586 g

f)

Formula of Naphtahlene = C10 H8

so molecular mass = (10*12 ) + (8*1) = 128 g

It is close to 126.84586 g

so the result of Part e is consistent with formula

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