Question

A solution is prepared by dissolving 5.00 g aspirin (C9H8O4) in 215 g chloroform (CHCl3). The boiling point of the solution is elevated by 0.47ºC over the bp of pure chloroform. Determine the value of Kb for CHCl3.

Answer 1

For the determination of Kb use the equation of elevation of boiling point :

T = K_b . m

where T is the change in the boiling point of the solent, K_b is the molal boiling point elevation constant , and m is the molal concentration of the solute in the solution.

T = 0.47 ^oC   (given)

Kb = ?

m= moles of solute / mass of solvent in Kg

= (5 g / 180.157 g/mol) / 0.215 Kg

= 0.129 mol/Kg

Now find K_b by using the equation and putting the values:

K_b = 0.47 ^oC / 0.129 mol/kg

= 3.643 ^o C Kg mol^-1