Suppose that you add 24.3 g of an unknown molecular compound to 0.250 kg of benzene, which has a K f of 5.12 oC/m. With the added solute, you find that there is a freezing point depression of 3.14 oC compared to pure benzene. What is the molar mass (in g/mol) of the unknown compound

Solution :

We know that :

 and  

Then,   ..................(1)

Where,

= amount of solvent (in kg)

= amount of solute (in kg)

= molar mass of solute (g/mole)

= molality of solution (mole/kg)

Given :

= ,  

= 0.250 kg,   = 24.3 g

Then putting this values in the equation is (1),

 g/mole

So, the molar mass of the unknown compound is 158.49 g/mole.