chemistry 102 college

Question

When 32.1 mg of a compound is dissolved in enough water to make 11.2 mL of solution at 25°C, the solution has an osmotic pressure of 554 torr. Calculate the molar mass of this compound.

What is the osmotic pressure given in units of atmospheres?

Answer 1

Colligative properties:

π = iMRT

π = osmotic pressure = 554 torr x 1 atm / 760 torr = 0.729 atm

i = van't Hoff factor = 1 assuming the unknown is a non electrolyte

M = molarity = moles unknown / liter of solution = ?

R = gas constant = 0.0821 Latm/Kmol

T = temperature in K = 25C + 273 = 298K

Solving for M, we have ...

M = π / iRT = 0.729 / (1)(0.0821)(298)

M = 0.0298 moles / liter

Since we have only 11.2 mls, we can find the number of moles ...

11.2 mls x 1 L / 1000 mls x 0.0298 mol / L = 0.000334 moles of unknown

molar mass = g / mole = 32.1 g / 0.000334 moles

molar mass = 96,177 g/mole