o
M2 can be determined by the equation o
The values of Kf and Kb can be determined from the following relations .
R = gas constant
M1 = molar mass of the solvent
Δfus H = enthalpies for the fusion of the solvent .
Δvap H = enthalpies for the vaporization of the solvent .
Calculate the mass of ascorbic acid ( Vitamin C , C6H8O6 ) to be dissolved in 75 g of acetic acid to lower its melting point by 1.5 ° C .
Kf = 3.9 K kg mol -1 .
Sol . Mass of acetic acid , w1 = 75 g Molar mass of ascorbic acid ( C6H8O6 ),
M2 = 6 × 12 + 8 × 1 + 6 × 16 = 176 g mol - 1
Lowering of melting point , ΔTf = 1.5 K
= ( 1.5 x 176 x 75 ) / ( 3.9 x 1000 ) = 5.08 g ( approx )
Hence , 5.08 g of ascorbic acid is needed to be dissolved .