A hydrated salt is one in which the solid crystals contain water of crystallisation.

An anhydrous salt is one in which the solid crystals do not contain water of crystallisation.

The empirical formula is the smallest whole number ratio of atoms of each element within a compound.

For example, the empirical formula of N₂O₄ is NO₂.

The molecular formula is a formula that shows the exact number of atoms of each element within a compound.

For example, the molecular formula of water is H₂O, while ethane is C₂H₆. 

1. Assume a total sample size of 100 g, which allows the percentage values to be used directly as grams.
2. Calculate the moles of each element by dividing the mass of each element by its atomic mass.
3. Divide each mole value by the smallest mole number to simplify the ratio to its lowest form.
4. Construct the empirical formula using the simplified mole ratio obtained in step 3.

For example, a salt contains 36% calcium and 64% chlorine. Assuming a 100 g sample, the salt contains 36 g of Ca and 64 g of Cl. Dividing these masses by their respective atomic masses yields 0.90 mol of Ca and 1.8 mol of Cl. Dividing these mole values by the smaller value (0.9) gives a Ca:Cl ratio of 1:2. Thus, the empirical formula of the salt is CaCl₂.

1. Calculate the empirical formula mass by adding the atomic masses of all the atoms in the empirical formula.
2. Divide the compound's molecular mass by the empirical formula mass to determine the number of empirical formula units per molecule.
3. Multiply the number of atoms of each element in the empirical formula by the number of empirical formula units to obtain the molecular formula.

For example, an alkene with a molecular mass of 28 has an empirical formula of CH₂ and an empirical formula  mass of 14. By dividing 28 by 14 we find there are 2 x CH₂ formula units per molecule, so the molecular formula of the alkene is C₂H₄.

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Water of crystallisation refers to the water molecules that are incorporated within a crystal lattice.