Deducing Molecular Formula

General Chemistry • Chemical Compounds

Written by STEM Calculators Team Published October 16, 2025 Updated February 24, 2026

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Molecular Formula from Empirical Formula and Molar Mass

The molecular formula contains the actual number of atoms of each element in a molecule. It is an integer multiple of the empirical formula. To determine it, compute the empirical-formula molar mass, compare it to the experimental molar mass, and multiply all empirical subscripts by the resulting integer.

Method

Compute the empirical-formula molar mass by summing each element’s atomic mass times its subscript.
Divide the experimental molar mass by the empirical-formula mass to obtain a ratio.
Round the ratio to the nearest whole number (it should be close to an integer).
Multiply all empirical subscripts by this integer to get the molecular formula.

\[ \begin{aligned} M_{\mathrm{emp}} &= \sum n_i A_i \\ n &= \frac{M_{\mathrm{exp}}}{M_{\mathrm{emp}}} \\ \text{Molecular subscripts} &= n\cdot (\text{empirical subscripts}) \end{aligned} \]

Example

Empirical formula: CH₂O. Experimental molar mass: 180.16 g·mol⁻¹.

\[ \begin{aligned} M_{\mathrm{emp}} &= 1\cdot 12.011 + 2\cdot 1.008 + 1\cdot 15.999 \\ &= 30.026\ \mathrm{g\,mol^{-1}} \end{aligned} \] \[n = \frac{180.16}{30.026} \approx 6 \] \[\mathrm{CH_2O}\times 6 = \mathrm{C_6H_{12}O_6}\]

Molecular formula: C₆H₁₂O₆.

Notes

If the ratio is not close to an integer, recheck the empirical formula or the measured molar mass.
Use consistent atomic masses for best agreement.

Frequently Asked Questions

How do you determine a molecular formula from an empirical formula and molar mass?

Compute the empirical-formula molar mass Memp, then find n = Mexp / Memp. Multiply every empirical subscript by the integer n to get the molecular formula.

What is the difference between empirical formula and molecular formula?

The empirical formula is the simplest whole-number ratio of atoms. The molecular formula gives the actual number of each atom in a molecule and is an integer multiple of the empirical formula.

Why should the ratio Mexp / Memp be close to a whole number?

A real molecule must contain whole numbers of atoms, so the molecular formula must be an integer multiple of the empirical formula. If the ratio is not close to an integer, the measured molar mass or the empirical formula may be incorrect.

What should I do if the multiplier is not an integer after rounding?

Recheck the empirical formula reduction and confirm the experimental molar mass value and units. Small deviations can come from measurement or rounding, but large deviations usually indicate incorrect inputs.

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