A solution of urea (molar mass 56 g mol^–1) boils at 100.18 ºC at atmospheric pressure. If K_f and K_b for water are 1.86 and 0.512 K kg mol^–1 respectively, the above solution will freeze at:

1. –6.54 ºC

2. –0.654 ºC

3. 6.54 ºC

4. 0.654 ºC

The boiling point of benzene is 353.23 K. When 1.80 g of a non-volatile solute was dissolved in 90 g of benzene, the boiling point is raised to 354.11 K. The molar mass of the solute is-

K_b for benzene is 2.53 K kg mol^–1

1. 55 g mol^–1
2. 64 g mol^–1
3. 58 g mol^–1  
4. 68 g mol^–1