Molecular formula

The molecular formula gives the actual number of atoms in one molecule, and in SPM Chemistry it almost always follows an empirical-formula step. Our online one-to-one teachers make sure you never stop at the empirical formula when the question, using the relative molecular mass, is really asking for the molecular one.

When you use it

Use this whenever a question gives the empirical formula (or the data to find it) together with the relative molecular mass, Mr, and asks for the molecular formula. The Mr is the extra clue that tells you how many empirical units make up one real molecule.

The method

1. Find (or use) the empirical formula.
2. Work out the empirical formula mass by adding the Ar values in one empirical unit.
3. Find n = relative molecular mass / empirical formula mass.
4. Multiply every subscript in the empirical formula by n.

In symbols, molecular formula = (empirical formula) with each subscript x n.

Units

Both the relative molecular mass and the empirical formula mass are ratios with no unit, so n itself has no unit and should come out as a whole number.

Worked example 1 (easy), from CH₂

A compound has the empirical formula CH₂ and a relative molecular mass of 42. Find the molecular formula. (Ar: C = 12, H = 1.)

• Step 1, empirical formula mass: 12 + (2 x 1) = 14.
• Step 2, n = 42 / 14 = 3.
• Step 3, molecular formula = C₃H₆.

Worked example 2 (medium), glucose

A sugar has the empirical formula CH₂O and a relative molecular mass of 180. Find the molecular formula. (Ar: C = 12, H = 1, O = 16.)

• Step 1, empirical formula mass: 12 + (2 x 1) + 16 = 30.
• Step 2, n = 180 / 30 = 6.
• Step 3, molecular formula = C₆H₁₂O₆.

Worked example 3 (SPM level), from percentage and Mr

A compound is 92.3 % carbon and 7.7 % hydrogen by mass, with a relative molecular mass of 78. Find the molecular formula. (Ar: C = 12, H = 1.)

• Step 1, empirical formula: C = 92.3 / 12 = 7.69; H = 7.7 / 1 = 7.7; ratio 1 : 1, so empirical formula = CH.
• Step 2, empirical formula mass: 12 + 1 = 13.
• Step 3, n = 78 / 13 = 6.
• Step 4, molecular formula = C₆H₆.

This two-stage question, find the empirical formula, then scale to the molecular formula, is the full SPM version.

Common traps

• Stopping at the empirical formula when the Mr shows the molecule is a multiple of it.
• Using the wrong empirical formula mass, which makes n wrong.
• Getting a non-whole n and rounding it carelessly rather than rechecking the earlier working.
• Multiplying only one subscript instead of every subscript by n.
• Forgetting that you cannot find a molecular formula without the relative molecular mass.

How we help

Our teachers link this straight onto the empirical-formula method, then add the Mr step, so the two stages feel like one smooth calculation. Lessons are one-to-one, taught in English, from RM50 per hour, with a paid one-hour trial so you can see the method before committing. Once you can move from data, to empirical formula, to molecular formula in one run, this whole family of questions becomes some of the most reliable marks in the mole chapter.