How to find the molecular formula from the empirical formula
Molecular formula = empirical formula × n, where n = molar mass ÷ empirical formula mass.
- Calculate the empirical formula mass from the atomic masses.
- Divide the measured molar mass by the empirical formula mass.
- Round the result to the nearest whole number, n.
- Multiply every subscript in the empirical formula by n.
Empirical formula vs molecular formula
An empirical formula shows the simplest whole-number ratio of atoms. A molecular formula shows the actual number of atoms in one molecule.
| Compound | Empirical formula | Molecular formula | n |
|---|---|---|---|
| Glucose | CH2O | C6H12O6 | 6 |
| Benzene | CH | C6H6 | 6 |
| Hydrogen peroxide | HO | H2O2 | 2 |
| Dinitrogen tetroxide | NO2 | N2O4 | 2 |
| Water | H2O | H2O | 1 |
Molecular formula problems
Problem 1
The empirical formula is CH2O and the molar mass is 180.16 g/mol. Find the molecular formula.
- Empirical formula mass = 12.011 + 2 × 1.008 + 15.999 = 30.026 g/mol.
- n = 180.16 ÷ 30.026 = 6.00.
- Multiply each subscript by 6.
Answer: C6H12O6
Problem 2
The empirical formula is CH and the molar mass is 78.11 g/mol. Find the molecular formula.
- Empirical formula mass = 12.011 + 1.008 = 13.019 g/mol.
- n = 78.11 ÷ 13.019 = 6.00.
Answer: C6H6
Problem 3
The empirical formula is NO2 and the molar mass is 92.01 g/mol. Find the molecular formula.
- Empirical formula mass = 14.007 + 2 × 15.999 = 46.005 g/mol.
- n = 92.01 ÷ 46.005 = 2.00.
Answer: N2O4
Problem 4
The empirical formula is CH2 and the molar mass is 56.11 g/mol. Find the molecular formula.
- Empirical formula mass = 12.011 + 2 × 1.008 = 14.027 g/mol.
- n = 56.11 ÷ 14.027 = 4.00.
Answer: C4H8
How to use this calculator
- Enter the empirical formula.
- Enter the measured molar mass in g/mol.
- Calculate to get the whole-number multiplier and the molecular formula.
Formula and variables
multiple = measured molar mass / empirical formula mass
| Symbol | Meaning | Unit |
|---|---|---|
| M | measured molar mass | g/mol |
| Mₑ | empirical formula mass | g/mol |
| k | formula multiple | integer |
Worked example
An empirical formula CH2O and measured molar mass near 180.16 g/mol gives a multiplier of 6 and molecular formula C6H12O6.
Interpretation
The molecular formula is accepted only when the mass ratio is an integer within the stated tolerance.
Common mistakes
- Use the empirical formula, not a percentage list.
- Do not force a non-integral measured mass ratio.
Frequently asked questions
How do you find the molecular formula from the empirical formula?
Divide the molar mass by the empirical formula mass. Multiply each subscript in the empirical formula by that whole number.
What information does a molecular formula provide?
A molecular formula gives the actual number of atoms of each element in one molecule. It also gives the molar mass.
How is a molecular formula different from an empirical formula?
An empirical formula gives the smallest ratio. A molecular formula gives the actual atom counts.
Why can the calculator reject a measured molar mass?
The molecular mass must be an integer multiple of the empirical formula mass within the stated tolerance.
Does this tool identify a formula from exact mass alone?
No. This tool requires an empirical formula. Exact-mass formula searches need element and charge constraints.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.