A mole ratio of 1 : 1.5 becomes 2 : 3 when you multiply both terms by two. Use the resulting whole-number ratio for the empirical formula. The 1.5 is part of the ratio, so rounding it to 2 would change the composition.
Start with the amount in moles of each element. Divide by the smallest amount to see the ratio clearly. If that ratio contains a half-integer, scale every term together and check that the result is in its simplest form.
- Convert each element’s mass to moles before comparing the amounts.
- Divide every amount by the smallest amount in moles.
- Multiply all terms by the same number to remove a fractional part.
CHAPTER 1 OF 4 · Back to contents
What an empirical formula represents
An empirical formula gives the simplest whole-number ratio of the elements in a compound. The numbers refer to atoms, so you first need a mole ratio. Equal masses of different elements generally contain different numbers of atoms.
For each element, use amount in moles = mass ÷ molar mass. Record the element symbols beside your values so you can transfer the final ratio to the formula in the right order.
CHAPTER 2 OF 4 · Back to contents
Worked example | The ratio is 1 : 1.5
A compound contains 2.80 g of nitrogen and 4.80 g of oxygen. Use relative atomic masses N = 14.0 and O = 16.0. Nitrogen gives 2.80 ÷ 14.0 = 0.200 mol. Oxygen gives 4.80 ÷ 16.0 = 0.300 mol.
Divide both values by 0.200 to get N : O = 1 : 1.5. Multiply both by two to get 2 : 3. The empirical formula is N2O3.
Check that 2 : 3 gives the same relative amounts. Dividing both terms by two returns 1 : 1.5. The ratio 1 : 2 would give more oxygen per nitrogen atom than the data support.
CHAPTER 3 OF 4 · Back to contents
Practise with 2.5 and find the molecular formula
A phosphorus oxide sample contains 6.20 g of phosphorus and 8.00 g of oxygen. Use P = 31.0 and O = 16.0. The amounts are 0.200 mol P and 0.500 mol O, giving 1 : 2.5. Multiply both by two to get P2O5.
If its relative molecular mass is given as 284, calculate the empirical formula mass first. It is 2(31.0) + 5(16.0) = 142. Since 284 ÷ 142 = 2, multiply both subscripts by two. The molecular formula is P4O10.
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
What if my ratio is close to 1.5?
Measurements and rounded data can give a nearby value such as 1.49. Keep enough digits during the calculation and check whether doubling gives values close to whole numbers.
What if the ratio includes about 1.33?
A value close to 1.33 can represent four thirds. Try multiplying the entire ratio by three and check whether all the terms are close to integers.
Do I need molecular mass to find the empirical formula?
The composition is enough for the empirical formula. Relative molecular mass lets you determine which whole-number multiple gives the molecular formula.
Contents · Official syllabus and next practice
Official syllabus and next practice
Read the official SEAB 2027 syllabus
Continue with finding oxygen mass by difference before calculating the mole ratio.