Question

Given:

mass of Biphenyl: 5.94g

mass of Benzoic Acid: 1.10g

mass of Biphenyl after boiling: 5.92g

mass of biphenyl and benzoic acid solution: 7.02g

Based on the data collected from colligative properties of solutions and freezing point depressions, calculate the following:

a) the observed molecular weight of benzoic acid in biphenyl.

b) the ratio of the observed molality to the initial molality. Why is the ratio not equal to one?

Answer 1

1)Biphenyl M.F. C12H10 hence calculated M.W. = 154. Benzoic acid M.F. C7H6O2 hence calculated M.W. = 122.

2) Given data: mass of Biphenyl = 5.92g and mass of benzoic acid 1.1g. We can find out Van’t Hoff factor (i).

i = Delta T_f / (K_fx m) which would give value ½

Observed Molecular weight of Benzoic acid = (½)x Actual M.W. = (½)x 122 = 61

This means molality will be half of the actual. This is because in Biphenyl the Benzoic acid dimerizes and therefor it counts I molecule les form every 2. Hence ration is not 1.

This is the abnormal behavior of solute in solvent.