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Molar Mass from Osmotic Pressure

 

molar mass= grams /moles  , so we need to find the grams and divide that # by the number of moles

1.  Set up this equation and place the grams on top. Now you need to find the moles to complete the problem.

molar mass=

grams
moles

2.  Find the Molarity using osmotic pressure and temperature

p=MRT

so

M=

   p  
 RT

3. Find the moles of solute from molarity by multiplying by the liters of solution.

Molarity (M)=

moles solute
liters of solution

so

Molarity (M)  x liters of solution= moles of solute

4. Now that you have the moles, plug it back into the equation from step 1 and solve for molar mass.

molar mass=

grams
moles

Example-Molar Mass from Osmotic Pressure

An aqueous solution contains 0.97g/L of an organic compound. The osmotic pressure of this solution is 62.9 torr, at 25C. What is the molar mass of this compound?

 

1.  Set up this equation and place the grams on top. Now you need to find the moles to complete the problem.

assume 1 L

molar mass=

0.97grams
   X moles

2.  Find the Molarity using osmotic pressure and temperature

p=MRT

p=62.9 torr x

1 atm =0.0828 atm
760 torr

M=

0.0828 atm

= 0.00338M
 (0.0821 L atm/mol K) (298K)

3. Find the moles of solute from molarity by multiplying by the liters of solution.

0.0338M=0.0338mol/L since assumed 1 L

0.0338mol

4. Now that you have the moles, plug it back into the equation from step 1 and solve for molar mass.

molar mass=

0.97g =287g/mol
0.00338mol

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