Question

The experimentally determined molar mass for what type ofsubstances is always lower than the true value when water is used as solvent. Explain. Give one example of such a substance and one example of a substance which does not show a large variation from the true value.

Answer 1

When there is dissociation of solute into, in dilute solutions (ignoring interionic attractions) the number of particles increases. As the value of colligative properties depends on the number of particles of the solute, the experimentally observed value of colligative property will be higher than the true value, therefore the experimentally determined (observed) molar mass is always lower then the true value.
for KCl (electrolyte) the experimentally determined molar mass is always lower than the true value when water is used as solvent.
Glucose (non-electrolyte) does not show a large variation from the true value.