Empirical & Molecular Formula, and Hydrated Salts Calculations (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about empirical and molecular formulae, and hydrated salts calculations.

A water of crystallisation calculation is an empirical formula calculation with the salt and the water in place of the elements, so the same method gives you both formulae.

An empirical formula is the simplest whole number ratio of the elements in a compound, and a molecular formula is the actual number of atoms of each element in a compound. You find an empirical formula by turning masses into moles and dividing by the smallest. A hydrated salt contains water of crystallisation, and finding how much water it holds uses that same calculation: instead of elements you start with the salt and the water, and instead of dividing by atomic masses you divide by formula masses. The rest of the working is identical.

We'll start with what the two formulae mean and how they differ, then go through how the calculation runs, and finish with water of crystallisation as the case where that calculation is adapted.

The molecular formula shows the number and type of each atom in a molecule. Ethanoic acid has the molecular formula C₂H₄O₂. The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit, so for ethanoic acid that is CH₂O.

Organic molecules often have different empirical and molecular formulae. Glucose is C₆H₁₂O₆ as a molecular formula and CH₂O as an empirical formula. Simple inorganic molecules are often the same in both: water is H₂O and carbon dioxide is CO₂ either way.

Ionic compounds do not have molecular formulae at all, because they form giant lattices rather than discrete molecules. The formula of an ionic compound is a formula unit, and that is the same as its empirical formula. Magnesium chloride is MgCl₂, which gives the ratio of magnesium ions to chloride ions in the lattice.

The empirical formula is the one you calculate from experimental data, and it is the ratio a hydrated salt calculation finds as well.

The empirical formula is calculated from the ratio of masses of each element in the compound. You may be given those masses directly, or given the percentage composition by mass. The working is the same either way.

Divide the mass of each element by its relative atomic mass to get the number of moles of each. Then divide every one of those mole values by the smallest of them, which gives you the ratio of the elements. If that ratio does not come out as whole numbers, multiply through until it does.

To get from the empirical formula to the molecular formula, work out the relative formula mass of the empirical formula, then divide the relative molecular mass of the compound by it. Multiply the number of each element in the empirical formula by that answer.

Masses to moles, then moles to a simplest ratio: that is the sequence a hydrated salt calculation reuses.

Water of crystallisation is water molecules that are chemically incorporated into the crystal structure of a compound. A compound that contains water is hydrated, and one that does not is anhydrous. In the formula, the water molecules are shown after a dot, so hydrated copper(II) sulfate is CuSO₄·5H₂O and the anhydrous form is CuSO₄.

Some compounds exist in more than one hydrated form. Cobalt(II) chloride can be CoCl₂·6H₂O or CoCl₂·2H₂O.

Heating a hydrated salt removes the water of crystallisation and leaves the anhydrous salt, and adding water to the anhydrous salt reverses it. That is what makes the degree of hydration something you can measure. Weigh the hydrated salt, heat it until it reaches a constant mass, and the decrease in mass is the mass of water lost.

From there it is the same calculation we did previously, with the salt and the water in place of the elements and formula masses in place of atomic masses. Divide each mass by its formula mass to get moles, divide by the smaller of the two, and the ratio of salt to water is the number in front of the H₂O.

AQA mark schemes award no credit for a final formula given without working shown. Students who jump straight to the molecular formula from the relative molecular mass can lose most of the marks even when the answer is correct.

When percentages are given for only some of the elements, check that they add up to 100% first. A recent AQA examiner report noted that a significant number of students missed finding a hidden element by subtraction before starting the empirical formula calculation.

The empirical formula is the simplest whole number ratio of the elements in a compound. The molecular formula is the actual number of atoms of each element.

You get the empirical formula by dividing each mass by its relative atomic mass and then dividing by the smallest result, and you get the molecular formula by scaling that up using the relative molecular mass.

Water of crystallisation is found the same way. The salt and the water take the place of the elements, formula masses take the place of atomic masses, and the ratio that comes out is the number of water molecules in the hydrated formula.