Empirical Formula Calculator

Empirical and molecular formula from mass percent or grams of each element, with every step shown.

Treat percentages as grams in 100 g, divide by atomic mass to get moles, divide by the smallest, and multiply to whole numbers. 40.0% C, 6.71% H and 53.3% O gives 3.33 : 6.66 : 3.33 = 1 : 2 : 1, so CH₂O; with a molar mass of 180 g/mol the molecular formula is C₆H₁₂O₆.

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How to use it

  1. Enter each element symbol with its mass percent (or grams).
  2. Add the compound's molar mass if you also want the molecular formula.
  3. Read the empirical and molecular formulas and the ratio table.

What you enter

What you get

Formula

nᵢ = massᵢ / Aᵢ | ratioᵢ = nᵢ / n(min) | molecular = (empirical)ₖ, k = M / M(empirical)

Worked example: iron oxide

Input: Fe 69.94%, O 30.06%

Result: Fe₂O₃

The 1 : 1.5 ratio is doubled to whole numbers.

Questions

How do you find the empirical formula from percentages?

Assume 100 g so percent equals grams, convert each to moles, divide all by the smallest number of moles, then multiply by a small whole number if any ratio ends in .5, .33 or .25.

What is the difference between empirical and molecular formula?

The empirical formula is the simplest whole-number ratio (CH₂O). The molecular formula is the actual count (C₆H₁₂O₆), a whole-number multiple of it.

How do I get the molecular formula?

Divide the compound's molar mass by the empirical formula mass, round to a whole number n and multiply every subscript by n.

What if a ratio comes out as 1.5?

Multiply every ratio by 2. For 1.33 or 1.67 multiply by 3, and for 1.25 by 4.

Limitations

More Chemistry

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.