Mass → amount
For percentages, imagine a 100 g sample. Divide each mass by the element's molar mass.
← All chemistry tools
01 / Divide 02 / Compare 03 / Scale
FORMULA CALCULATOR · LEARNING LABSmall ratios.
Small ratios.
Big discoveries.
Turn a sample into a chemical formula. See the recipe, follow the clues, then try it yourself.
THE SIMPLEST RECIPECH2O× 6 → C6H12O6Same proportions. A bigger molecule.
START WITH YOUR SAMPLE
What is it made of?
Choose an example or enter your own composition.
THE COMPOSITION STUDIO
Mass is only the first clue.
Heavier atoms contribute more mass. Divide by atomic mass to compare how many atoms there are.
FORMULA RECIPE
Counting tiles show composition, not bonds, shape or the arrangement of a crystal.
FOLLOW THE EVIDENCE
From measurements to formula
Find the ratio
Divide all amounts by the smallest. Multiply all ratios by the same number if fractions remain.
Build the molecule
An empirical formula gives the simplest ratio. A molecular formula needs a relative molecular mass too.
WATER DETECTIVES
Find the hidden water.
A hydrated salt contains water in its crystal structure. Compare relative formula masses to find x in salt·xH₂O.
These are relative formula masses, not the masses of two laboratory samples. Assumes all of the difference is water of crystallisation.
INSIDE THE CRYSTAL
One salt formula unit + ? water
Choose an example and count the water.
The tiles count water molecules per salt formula unit. They do not show the crystal structure.
Use + and − to choose each subscript. A 1 is left out when writing the formula.
How to use the models and rounding
These visuals explain composition; they are not molecular structures. The formula alone cannot identify a substance or show its bonding. Empirical formulas are inferred from complete elemental composition, assuming a pure compound.
The calculator looks for small whole-number ratios using multipliers up to 12, with a maximum deviation of 0.055 from a whole number. This supports typical rounded classroom data, but uncertain experimental results may need more analysis. Percentages must total within 0.5% of 100. Molecular and hydrate multipliers must be within 0.02 of a positive whole number.
Relative atomic mass Ar and relative formula/molecular mass Mr are dimensionless. An element's molar mass in g mol⁻¹ has the same numerical value as its Ar at the precision used here. Keep extra digits until the final ratio.
Keep exploring with the Moles Calculator or empirical formula guide.