How many moles of an unknown compound must be dissolved in 100 mL of water to yield an osmotic pressure of 28.1 mmHg at 20 °C (assume the unknown has negligible volume)?

Answer 1

π = iMRT

π = osmotic pressure = 28.1 mm Hg

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

M = molarity = moles/liter = ?

R = gas constant = 62.36 L-mmHg / K-mol

T = temperature in K = 20ºC + 273 = 293K

Solving for M, we have...

M = π / (i)(R)(T)

M = 28.1 / (1)(62.36)(293)

M = 0.00154 M = 0.00154 mols/liter

Since we have 100 ml solution (0.100 L), we can solve for moles...

0.00154 mol / L x 0.1 L = 0.000154 moles = 1.54x10^-4 moles