Empirical Formula

Find the empirical and molecular formula from mass percentages, molar mass or gas density, with a step-by-step solution.

Write the composition as “C 85.7, H 14.3” or “C 40%, H 6.7%, O” — one element may have no fraction, it gets the remainder to 100 %. You can enter masses of elements instead of percentages.

Test yourself

The empirical formula calculator finds the simplest formula from the mass percentages of the elements and, if the molar mass or gas density is known, the molecular formula.

How to derive a formula

Take 100 g of the substance — the masses of the elements in grams equal their percentages.
Find the amounts of atoms: n = w / A_r.
Divide all numbers by the smallest and, if needed, multiply by 2, 3… to get whole numbers — this is the empirical formula.
If the molar mass is given, divide it by the molar mass of the empirical formula and multiply the subscripts by the result.

Molar mass from density

Given	Molar mass
density relative to hydrogen	M = 2 · D_H₂
density relative to air	M = 29 · D_air
density at STP, g/L	M = 22.4 · ρ

Example. C 85.7 %, H 14.3 %, D_H₂ = 21. n(C) : n(H) = 7.14 : 14.3 = 1 : 2, empirical formula CH₂ (M = 14). M = 2 · 21 = 42, k = 42 / 14 = 3 — the formula is C₃H₆.