Empirical Formula Calculator
Enter each element's mass or mass percent to get the mole ratios and the simplest whole-number formula.
Enter each element's mass or mass percent in a sample (the unit doesn't matter -- only the proportion between rows does).
| Element | Mass | ÷ atomic weight = mol | ÷ smallest = ratio |
|---|---|---|---|
| C | 40 | 3.3303 | 1.000 |
| H | 6.7 | 6.6468 | 1.996 |
| O | 53.3 | 3.3315 | 1.000 |
Finding an Empirical Formula
Given each element's mass in a sample, divide each by that element's atomic weight to get moles of that element, then divide every result by the smallest mole value to get a ratio. Rounding those ratios to whole numbers gives the empirical formula -- the simplest atom ratio the sample's composition supports.
This only works from composition data (masses or percentages), which is exactly what a combustion analysis or similar lab technique produces -- it can't tell you the actual molecular formula without one more piece of information, the compound's real molar mass.
Worked example
A compound is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Find its empirical formula.
C: 40 / 3.3303 mol → ratio 1.00; H: 6.7 / 6.6468 mol → ratio 2.00; O: 53.3 / 3.3315 mol → ratio 1.00.
All three round cleanly to whole numbers, giving CH2O -- this is glucose's empirical formula (its actual molecular formula, C6H12O6, is six times this ratio).
Common mistake
Rounding a ratio like 1.5 straight to 2, instead of recognizing that every ratio needs doubling first. 1, 1.5, 1 doubles to 2, 3, 2 -- a real whole-number set -- while rounding the 1.5 alone gives a formula with the wrong relative amounts.
Keep going
- An empirical formula gives a ratio, not a molecular formula; once you know the real molar mass from other data, you can find the molecular formula as a whole-number multiple. Molar Mass Calculator
- Empirical formula analysis starts from element masses in a sample; stoichiometry takes a known formula's mole ratio and scales it to a different amount of reactant or product. Stoichiometry Calculator
Frequently Asked Questions
What is an empirical formula?
The simplest whole-number ratio of atoms in a compound, found from experimental mass data -- not necessarily the compound's actual molecular formula. Glucose's molecular formula is C6H12O6, but its empirical formula is CH2O, the same 1:2:1 ratio reduced to its simplest form.
Why divide by the smallest mole value?
Dividing every element's mole count by the smallest one scales the whole set down so the smallest becomes exactly 1, turning raw mole values into a comparable ratio -- the same reason you'd simplify a fraction like 6/12/6 to 1/2/1.
What if the ratios don't come out as whole numbers?
Real lab measurements always carry some error, so a ratio like 1.98 or 3.01 should be read as 2 or 3. But a ratio that lands clearly between two whole numbers (like 1.5) usually means every ratio needs to be multiplied by the same small number (2, for a ratio ending near .5) before rounding.
Can I use mass percent instead of actual mass?
Yes -- mass percent works exactly the same as actual mass in this calculation, since only the proportion between elements matters, not the absolute amount. A sample reported as "40% C, 6.7% H, 53.3% O" gives the identical ratio as 40 g, 6.7 g, 53.3 g.
How is empirical formula different from molecular formula?
The empirical formula is the simplest ratio; the molecular formula is the actual count of atoms in one real molecule, which is always a whole-number multiple of the empirical formula (1x, 2x, 3x...). You need the compound's real molar mass, separately, to tell which multiple applies.
Related Calculators
- Molar Mass Calculator — Atomic & Molecular
- Moles Calculator — Atomic & Molecular
- Atomic Mass Calculator — Atomic & Molecular
- Half-Life Calculator — Atomic & Molecular