MCAT General Chemistry Help?

[sharpieLIFE]

Hey Guys,
Can anyone lend a helping hand to me on this one? My tutor keeps insisting that a lot more gen chem and pchem should be studied. He gave me this one problem:

When 0.10 g of insulin is dissolved in 0.200 L of water, the osmotic pressure is 2.30 Torr at 20°C. What is the molar mass of insulin?

Thanks in advance!

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[cheesier]

pV=nRT

n = PV/RT

Convert 2.3 Torr to atm ===> 0.003 atm

n = (0.003)(0.2)/(8.3)(298) = 2.45 * 10^-7 moles

0.1 g/2.45*10^-7 moles ===> 400000 g/mol

It's pretty off from the actual value, but I think that's how you'd go about solving it.

 

[ClemsonFirst]

You should be using the osmotic pressure equation, not the ideal gas law. pi=iMRT where pi is osmotic pressure, i is van't Hoff factor (should be 1 for insulin since it does not dissociate), M is molarity, R is .0821 L atm / K mol, and T is temp.

Learn you equations for chem and physics. You'll need them.

 

[cheesier]

You should be using the osmotic pressure equation, not the ideal gas law. pi=iMRT where pi is osmotic pressure, i is van't Hoff factor (should be 1 for insulin since it does not dissociate), M is molarity, R is .0821 L atm / K mol, and T is temp.

Learn you equations for chem and physics. You'll need them.

Look carefully at that equation. Since i = 1, it is the ideal gas law, just with both sides divided by volume and some symbols changed. What did you think the osmolarity equation was derived from?

I did plug in the wrong R value though, so using 0.0821 should get you closer to the actual molar mass of insulin.

 

[ClemsonFirst]

Interesting. I've been tutoring chemistry for 3 years and never noticed that.