Question

In an experiment to determine the molar mass of dichlorobenzene using freezing point depression of solvent cyclohexane, a student uses 10 mL of cyclohexane(density 0.779 g/mL) and obtains an average freezing point of 5.75°C. she graphically determine is the new freezing point to be 2.8°C by setting equation of two lines equal to each other what is her delta Tf? if she uses a total of 0.2301 g of dichlorobenzene what is a calculated molecular weight for the solute? what is percent error from actual molecular weight of solute? is this a reasonable result?

Answer 1

dichlorobenzene = 147.01 g/mol approx

V = 10 mL of cyclohexane solvent

D = m/V; so

mass = D*V = 0.779 *10 = 7.79 g of cylohexane

mass in kg = 7.79*10^-3 kg of cyclohexane

m = 0.2301 g of solute

Tf(normal) = 5.75°C

Tf(mix) = 2.8°C

dT = 5.75-2.8 = 2.95 °C

for...

hexane = –20.2

so

dT = Kf*m

m = mol of soute / kg solvent

- 2.95 = -20.2 * mol of solute / ( 7.79*10^-3)

mol of solut e= 2.95 /20.2 * ( 7.79*10^-3) = 0.00113764

MW = mass/mol = 0.2301 /0.00113764 = 202.26 g /mol