What is the molecular weight they determined for this compound?

Question

In a laboratory experiment, students synthesized a new compound and found that when 11.72 grams of the compound were dissolved to make 233.4 mL of a benzene solution, the osmotic pressure generated was 4.51 atm at 298 K. The compound was also found to be nonvolatile and a non-electrolyte.

What is the molecular weight they determined for this compound?

Molar mass =  ________ g/mol

Answer 1

Osmotic pressure π = iMRT

π = 4.51 atm

i = van't Hoff factor = 1 (non electrolyte)

M = molarity = moles / L = see below calculation

R = gas constant = 0.0821 Latm/Kmol

T = temperature in Kelvin = 298K

Solving for M, we have ...

M = π / (i)(R)(T) = 4.51 / (1)(0.0821)(298)

M = 0.184 moles / L

0.184 mol / L x 0.2334 L = 0.0429 moles

molar mass = g / mol

molar mass = 11.72 g / 0.0429 moles

molar mass = 273 g/mol (3 sig. figs.)