Amount of Substance, Determination of Formulae

Full Notes

Understanding Empirical and Molecular Formulae

The empirical formula shows the simplest whole number ratio of atoms in a compound, while the molecular formula represents the actual number of atoms in a molecule.

Example Glucose (C₆H₁₂O₆)

Empirical formula = CH₂O (simplest ratio 1:2:1)
Molecular formula = C₆H₁₂O₆ (actual composition)

Determining the Empirical Formula

To find the empirical formula of a compound from mass or percentage composition:

1. Convert (percentage) mass of each element to moles using: Moles = Mass (g) ÷ Atomic mass (Ar)
2. Divide all mole values by the smallest number of moles.
3. Round to the nearest whole number to get the simplest ratio.
4. Write the empirical formula.

Ex

Worked Example

Find the empirical formula for the compound with a composition by mass of C 52.2%, H 13.0% and O 34.8%.

Determining the Molecular Formula

To find the molecular formula, use:

n = Molecular mass ÷ Empirical mass

Multiply the empirical formula by n to get the molecular formula.

Ex

Worked Example

The empirical formula of a compound is CH₂O, and its molar mass is 180 g/mol. Find the molecular formula.

1. Calculate the empirical formula mass.
2. Find n.
3. Multiply empirical formula by n.

Water of Crystallisation

Hydrated compounds contain water molecules as part of their crystal structure. The water molecules are trapped between particles in the solid.

Water of crystallisation refers to this water, shown in the formula as “•xH₂O”, where x is the moles of water in the solid compared to moles of compound.

For Example Copper(II) sulfate

A common hydrated form of copper(II) sulfate is CuSO₄•5H₂O (s).
This formula tells us that for every one mole of CuSO₄ in the solid crystal, there are also 5 moles of H₂O molecules.

Anhydrous compounds contain no water.

Example Anhydrous copper(II) sulfate

CuSO₄(s) is anhydrous as the formula contains no water of crystallisation.

To find the formula of a hydrated salt:

• Use mass data to calculate moles of salt and water lost.
• Find the molar ratio of anhydrous salt to water.

Summary

• The empirical formula shows the simplest whole number ratio of atoms in a compound.
• The molecular formula represents the actual number of atoms in a molecule.
• We can calculate empirical and molecular formulae from mass or percentage data and molar mass.
• Hydrated salts contain water of crystallisation, shown as •xH₂O in their formulas.