Answer
Molar mass is the actual mass of one mole of that compound. Empirical formula mass is the mass of one mole of the empirical formula compound when the subscripts are written in the lowest possible ratio.
Molar mass and empirical formula mass are equal when the molecular formula is identical to the empirical formula for that compound. Molar mass and empirical formula mass are different when the molecular formula is not identical to the empirical formula.
When different, the molar mass is equal to a whole number multiple of the empirical formula mass equal to the whole number ratio of the molecule's subscripts with the empirical formula's corresponding subscripts.
Work Step by Step
Molar mass is the measured mass of one mole of that substance where the subscripts are not necessarily reduced. Empirical formula mass, however, is always reduced to the lowest possible integer subscripts. As a result, molar mass and empirical formula mass are equal only when the molecule's formula is already reduced and therefore identical to the empirical formula. Conversely, if the molecular formula is a multiple of the empirical formula's subscripts, the molar mass would differ from the empirical formula's mass by a integer ratio.
