Molecular Formula (Higher Tier)

Definition of Molecular Formula

The molecular formula shows the actual number of atoms of each element in a molecule. It gives the exact composition of a molecule, unlike the empirical formula which only shows the simplest whole-number ratio of atoms.

For example, the molecular formula of glucose is C₆H₁₂O₆, which tells us there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms in one molecule of glucose.

Calculating Molecular Formula

To find the molecular formula, you need two pieces of information:

• The relative molecular mass (Mr) of the molecule (usually given or measured experimentally)
• The empirical formula of the compound (simplest whole-number ratio of atoms)

The molecular formula is a whole-number multiple of the empirical formula. To calculate it:

1. Calculate the Mr of the empirical formula.
2. Divide the Mr of the molecular formula by the Mr of the empirical formula to find the whole number factor.
3. Multiply the subscripts in the empirical formula by this factor to get the molecular formula.

For instance, if the empirical formula is CH₂ and the Mr of the molecule is 56, first find the Mr of CH₂:

$\text{Mr of CH}_2 = 12 + (2 \times 1) = 14$

Then, find the factor:

$\frac{56}{14} = 4$

Multiply the subscripts by 4:

C: $1 \times 4 = 4$, H: $2 \times 4 = 8$

So, the molecular formula is C₄H₈.

Using Molecular Formula in Calculations

The molecular formula allows you to calculate the mass of a substance when you know the number of moles, or vice versa, using the relative molecular mass (Mr).

The formula linking mass, moles, and Mr is:

$$\text{Mass (g)} = \text{Number of moles} \times \text{Mr}$$

For example, if you have 3 moles of a substance with Mr = 20, the mass is $3 \times 20 = 60$ g.

You can use the molecular formula to:

• Calculate the Mr by adding the relative atomic masses (Ar) of all atoms in the molecular formula.
• Find the mass of a given number of moles of the substance.
• Relate the molecular formula to chemical equations to understand the amounts of substances reacting or produced.

For example, to find the mass of 0.5 moles of water (H₂O), first calculate Mr:

$\text{Mr of H}_2\text{O} = (2 \times 1) + 16 = 18$

Then calculate mass:

$\text{Mass} = 0.5 \times 18 = 9 \text{ g}$