Answer to Question #187096 in Physical Chemistry for Ahmad

Question

An organic compound is known to be non-volatile and non-electrolyte. A 0.5-g sample is dissolved in water and diluted to 160 mL. The osmotic pressure is measured as 0.05 atm at 30°C. The approximate mass number for this compound is 1500 g/mol. From the data provided in this problem and knowing that the density of the solution is 1.00 g/mL. (a) Calculate the freezing point of the solutions. (b) Determine if the freezing point change would be a good way to determine the molecular mass of the compound. (c) Would the boiling point change be a better determining factor than the freezing point change?

Answer 1

(a)

ΔT = Kf x m

m – molality

Kf = 1.86 for water solutions

160 mL = 0.160 L or 0.160 kg (for density = 1)

ΔT = 1.86 x  $(0.5 / 1500) / 0.160$ = 0.003875

Tf = 0⁰ – 0.003875 = - 0.003875 ⁰C

b)

It’s not a good way to determine the molecular mass of the compound because the temperature change is hardly measurable. The result would be inaccurate.

c)

For the boiling point method this set of data is not good as well. The concentration of the compound is too low.