Empirical Formula Calculator: From Percent Composition or Mass, with the Molecular Formula

Find the empirical and molecular formula of a compound from its composition.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose up to four elements and enter the percent or mass of each.

Enter the molar mass of the compound if you want the molecular formula.

The calculator divides by the atomic masses, divides by the smallest, multiplies to whole numbers where needed and gives the formula.

Formulas used

  • n = \frac{m}{A_r}
  • \text{ratio} = \frac{n}{n_{min}}
  • k = \frac{M}{M_{empirical}}

Worked example

With the default numbers in the calculator above:

  1. Moles of each element: divide the mass (or percent) by the atomic mass: C 3.33 mol; H 6.65 mol; O 3.33 mol
  2. Divide each by the smallest number of moles: C 1.00; H 2.00; O 1.00
  3. Empirical formula (the simplest whole number ratio): CH2O
  4. Empirical formula mass: 30.03 g/mol
  5. Molecular formula multiplier: the molar mass divided by the empirical formula mass: 5.99
  6. Molecular formula: C6H12O6 (the empirical formula times 6)

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: First element Fe; Its percent or mass 69.94; Second element O; Its percent or mass 30.06; Third element none (not used); Its percent or mass 0; Molar mass of the compound, if known (0 = skip the molecular formula) 0 g/mol.

  1. Moles of each element: divide the mass (or percent) by the atomic mass: Fe 1.25 mol; O 1.88 mol
  2. Divide each by the smallest number of moles: Fe 1.00; O 1.50
  3. Multiply by 2 to get whole numbers (the ratios are not whole yet): multiplier 2
  4. Empirical formula (the simplest whole number ratio): Fe2O3
  5. Empirical formula mass: 159.69 g/mol

Common questions

What is an empirical formula?

The simplest whole number ratio of the atoms in a compound. For glucose, C6H12O6, it is CH2O.

What if the ratio is 1.5?

Multiply all the ratios by 2 (or by 3 for thirds) to make them whole numbers. Iron oxide with a ratio of 1 to 1.5 is Fe2O3.

How do I get the molecular formula?

Divide the molar mass by the empirical formula mass and multiply the empirical formula by the result.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: IUPAC standard atomic weights (abridged). Any first-year chemistry text.

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