The experimentally determined molar mass for what type of substances 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.
When there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value. Obviously, during the dissociation of ionic solute into ions, the number of particle increases in dilute solutions (ignoring interionic attractions).
As colligative properties depend on the number of particles of solute, the experimentally observed value would be higher than true value.
So, experimentally determined observed molar mass is always lower than true value. The ratio between observed colligative property and calculated value is represented as ‘i’ and is called Vant Hoff factor.
In ionic compounds like KCl, NaCl, MgSO4, experimentally determined molar mass is always lower than true value.
Glucose being a covalent compound does not show much variation from the true value.