The freezing point depression of a solution made by the addition of a nonionic u

Question

The freezing point depression of a solution made by the addition of a nonionic unknown solute to benzophenone was used to determine the molar mass of the unknown. Data was collected for a solution containing 500. g of of benzophenone with 5.0 ml of the unknown solute added. The density of the unknown solute was 6.18 g/ml. The K_f of benzophenone is 9.8 degree C M^-1. The freezing point of the benzophenone was determined to be 7.19 degree C. The freezing point of benzophenone with the unknown added was determined to be -0.98 degree C. (i) Determine the freezing point depression, Delta T_f (ii) Calculate the Molality of the solution, m (iii) calculate the molar mas of the nonionic unknown

Explanation / Answer

(i) Freezing point of pure Benzophenone = 7.19C

Freezing point of benzophneone with the unknown added = -0.98C

Thus,depression in freezing point,delta Tf = -0.98C - 7.19C = 8.17C

(ii) Depression in freezing point is expressed by:-

Delta tf = m.Kf

Delta tf = 8.17C

Kf = 9.8C/M

m = Delta Tf/Kf

m = 8.17C/9.8C/M

m = 0.834m

(iii) Molality = moles of solute/mass of solvent in kgs

Mass of benzophenone = 500g = 0.5kg

molality = 0.834

so,moles of solute = 0.834moles/kg*0.5kg = 0.417

Density of the unknown solute = 6.18g/mL

Volume of the solute = 5 mL

since Mass = Density * Volume = 6.18g/mL * 5 mL = 30.9g

now.moles of solute = Mass/Molar Mass

Molar Mass = Mass/moles = 30.9g/0.417mol = 74.1g/mol

Molar Mass = 74.1g/mol

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