Question

You have a 150mL sample of an aqueous solution at 25C. It contains 15.2mg of an unknown nonelectrolyte compound. If the solution has an osmotic pressure of 8.44 torr, what is the molar mass of the compound

Answer 1

The molar mass of the compound:

If the solution has an osmotic pressure of 8.44 torr, then the molar mass of the unknown non-electrolyte is 223.14 g.

What is osmosis?

• Osmosis is defined as the flow of solvent molecules through semi-permeable membrane.
• Osmotic pressure is the pressure applied to stop the flow of solvent molecules.
• It is a colligative property that means osmotic pressure depends on the number of solute particles .

Therefore,

π$V=inRT$  ( for electrolytes)

Where,   π= Osmotic pressure

i = Van 't Hoff factor

n= moles

R= Gaseous constant = 62.363577 L torr $mol^{-1}K^{-1}$

T= Temperature

V= Volume of solution

Given:

T= 298K

V= 150 mL= 0.150 L

Given mass of unknown electrolyte= 15.2 mg = 15.2 x  $10^{-3}$ g

Osmotic pressure= 8.44 torr

Molar mass= ?

For non-electrolytes:

πV = n RT

πV=$\frac{m}{M}$RT

Calculations:

Putting the given values in the formula:

8.44 x 0.150 =15.2 x $10^{-3}$/ M x 62.36 x 298

1.266 = 282.5/M

M = 282.5/1.266

M = 223.14 g

Therefore,

The molar mass of the unknown non-electrolyte is 223.14g.

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