Homework Help: Chem

Posted by Anonymous on Thursday, February 2, 2012 at 9:46pm.

An aqueous solution containing 21.6 mg of Vasopressin in 100 mL solution produces Osmotic Pressure of 3.70 mmHg at 25°C. Calculate the Molecular Mass of Vasopressin.

Here's what I started working on, but then I got stuck.

21.6 mg = 0.0216 g
v = 100 mL = 0.1 L
ð = 3.70 mmHg = 0.004 atm
T = 25°C + 273.15 K = 298.15 K

ð = MRT
M = RT/ð
M = [(0.0821 L-atm/mol-K)(298.15 K)]/0.004 atm

I kept getting a weird answer for moles. ~ 6.12E10^3

M = (??? moles)/L (0.1 L)
M = ???
MM = (0.0216 g/ ???) =

But if I were to use 6.12E10^3, I wouldn't get the molar mass of Vasopressin, which I calulated using the Chemical Formula in order to double-check myself. Can you please help me out? It would be greatly appreciated! :)

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