Boiling point is least for

;  where       ;              Thus greater the concentration of the solution,higher will be the B.P                   solution              net concentration                  0.1m urea                0.1 X 1=0.1                   0.2m urea               0.2 X 1=0.2                    0.1m NaCl              0.1 X 2=0.2                     0.2m MgCl2               0.2 X 3=0.6