. You prepare a solution by adding 20.0 g Urea [(NH2)2CO] to 125 g water at 25.0oC & vapor pressure 23.76 torr. The solution vapor pressure is 22.67 torr. Calculate the molecular weight of urea. XH2O = moles H2O = Xsolute = moles urea = molar mass = calculations:

Answer : The molecular wight of urea is, 59.9 g/mol

Mole fraction of water and urea is, 0.95 and 0.051 respectively.

Explanation :

According to the relative lowering of vapor pressure, the vapor pressure of a component at a given temperature is equal to the mole fraction of that component of the solution multiplied by the vapor pressure of that component in the pure state.

Formula used :

where,

= vapor pressure of the pure solvent water = 23.76 torr

= vapor pressure of the solution  = 22.67 torr

= mole fraction of solute  (urea)

n is the number of moles

= mass of urea = 20.0 g

= mass of water = 125 g

= molar mass of urea = ?

= molar mass of water = 18 g/mol

Now put all the given values in the above expression, we get:

Now we have to calculate the moles of and urea.

and,

Now we have to calculate the mole fraction of and urea.

and,