Question

Working on-board a research vessel somewhere at sea, you have (carefully) isolated 12.5 micrograms (12.5 ×10–6 g) of what you hope is pure saxitoxin (a non-electrolyte) from a poisonous (and quite cross) puffer fish. You dissolve this sample in 3.10 mL of water and determine that the osmotic pressure of the resulting solution is 0.236 torr at 19oC (760 torr = 1.00 atm). What is the molar mass of the compound? 15 pts.

Answer 1

Temperature = 19 C = 19 + 273 K = 292 K

given

osmotic pressure = 0.236 torr

osmotic pressure = 0.236 torr x 1 atm / 760 torr

osmotic pressure = 3.105 x 10-4 atm

now

we know that

for a non electrolyte

osmotic pressure = CRT

3.105 x 10-4 atm = C x 0.0821 ( L atm /mol K) x 292 K

C = 1.2953 x 10-5 mol /L

now

concentration (C) = moles of compound / volume of solution (L)

moles of compound = volume of soluton (L) x concentration

moles of compound = 0.0031 L x 1.2953 x 10-5 mol /L

moles of compound = 4.01545 x 10-8 mol

now

molar mass = mass / moles

molar mass of compound = 12.5 x 10-6 g / (4.01545 x 10-8 mol)

molar mass of compound = 311.3 g/mol