What if the ratio doesn't round to a whole number?

If you get ratios like 1.5, 1.33, or 1.25, multiply all ratios by 2, 3, or 4 respectively to get whole numbers. For example, a ratio of 1:1.5 becomes 2:3 when multiplied by 2.

How do I get the molecular formula from the empirical formula?

Divide the molecular mass (from mass spectrometry or other data) by the empirical formula mass. This gives you the multiplier n. Multiply all subscripts in the empirical formula by n.

Can I use percent composition directly?

Yes. Assume a 100 g sample, so the percentages become grams directly. Then convert those grams to moles and proceed as usual.

Calculadora de Fórmula Empírica

Encuentra la razón de números enteros más simple de los elementos a partir de la masa o composición porcentual.

Encuentra la razón de números enteros más simple de los elementos a partir de la masa o composición porcentual.

Masa del elemento 1 (g)

Masa molar del elemento 1 (g/mol)

Masa del elemento 2 (g)

Masa molar del elemento 2 (g/mol)

Formula

Divide each mole value by the smallest, then round to nearest whole numbers

What is This Tool?

The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. It is determined by converting masses to moles, dividing by the smallest mole value, and rounding to whole numbers. The molecular formula is a whole-number multiple of the empirical formula.

Formula

1. Convert mass of each element to moles: n = mass / molar mass
2. Divide each by the smallest mole value
3. Round to nearest whole numbers
4. These are the subscripts in the empirical formula

Worked Examples

1. Compound with C and H

Given: 40.0 g C (12.01 g/mol), 6.7 g H (1.008 g/mol)

mol C = 40.0 / 12.01 = 3.33

mol H = 6.7 / 1.008 = 6.65

Divide by smallest (3.33): C = 1, H = 2

Result: Empirical formula: CH2

2. Compound with Na and Cl

Given: 46.0 g Na (22.99 g/mol), 71.0 g Cl (35.45 g/mol)

mol Na = 46.0 / 22.99 = 2.00

mol Cl = 71.0 / 35.45 = 2.00

Ratio = 1 : 1

Result: Empirical formula: NaCl

Frequently Asked Questions