Question

the vapour pressure of water is 92mm at 323k . 18.1g of urea are dissolved in 100 g of water the vapour pressure is reduced by 5mm . calculate the molar mass of urea​

Answer 1

Answer: The molar mass of urea is 60 g/mol

Solution:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

$\frac{p^o-p_s}{p^o}=\frac{w_2M_1}{w_1M_2}$

where,

$p-p^o$ = lowering in vapour pressure= 5 mm

$p^0$= vapor pressure of pure solvent (water) = 92 mm

$w_2$ = mass of solute  (urea) = 18.1 g

$w_1$ = mass of solvent (water) = 100 g

$M_1$ = molar mass of solvent (water) = 18 g/mole

$M_2$ = molar mass of solute (urea) = M g/mole

Now put all the given values in this formula ,we get the vapor pressure of the solution.

$\frac{5}{92}=\frac{18.1 \times 18}{100\times M}$

$M=60g/mol$

Therefore, the molar mass of urea is 60 g/mol