Formula Making of compound

Define 'formula mass' of a compound.

Define water of crystallisation. Give the chemical formula for two compounds as examples. how can it be proved that the water of crystallisation makes a difference in the state and colour of the compounds?

The formula of the compound is :

Define the formula of a compound

The molecular formula of the compound

Define empirical formula of a compound.

Empirical formula of a compound is CH_(3)N . If the empieical formula weight is equal to one-fourth of its vapour density, find out the molecular formula of the compound.