An empirical formula calculator finds the simplest whole-number ratio of atoms in a compound from mass percent composition data. Enter the percentage of each element (as measured by combustion analysis or spectroscopy), and the tool converts them to mole ratios to give you the empirical formula — and the molecular formula if you provide the molecular weight.
Element Composition
Quick examples:
Results
Empirical Formula
Molecular Formula
Step-by-Step Calculation
| Element | Mass % | Grams (in 100g) | Atomic Mass | Moles | Mole Ratio | Subscript |
|---|
How to Use the Empirical Formula Calculator
The empirical formula calculator converts mass percent composition data into a chemical formula. This is the fundamental step in identifying unknown compounds from combustion analysis data or solving percent composition problems in general chemistry courses.
Step 1: Enter Element Percentages
For each element in the compound, enter the element symbol and its mass percentage. Start with 2 rows (provided by default) and click "Add Element" if the compound has more elements. The tool shows a running total so you can verify percentages sum to 100%. If the total is under 100% and you suspect oxygen, you can either add oxygen manually or let the calculation proceed with the given elements.
Step 2: Optional Molecular Weight
If you know the compound's molecular weight (from mass spectrometry or other data), enter it in g/mol. The calculator will determine the molecular formula multiplier n = MW / (empirical formula mass) and show the full molecular formula.
Step 3: Read the Results
The results show the empirical formula rendered with chemical notation, followed by a step-by-step table. Each row shows the mass, moles, mole ratio, and final subscript for each element.
Worked Example: Finding the Empirical Formula of Glucose
A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen. What is its empirical formula?
- C: 40.0g ÷ 12.011 g/mol = 3.330 mol
- H: 6.7g ÷ 1.008 g/mol = 6.647 mol
- O: 53.3g ÷ 15.999 g/mol = 3.331 mol
- Divide by smallest (3.330): C=1.000, H=1.996≈2, O=1.000
- Empirical formula: CH₂O
With molecular weight 180 g/mol: n = 180 / 30.026 = 5.99 ≈ 6, giving molecular formula C₆H₁₂O₆ (glucose).
Handling Non-Integer Ratios
Real data often produces ratios like 1.5, 1.33, or 1.25 due to rounding. The algorithm multiplies all ratios by 2 (for .5), 3 (for .33/.67), or 4 (for .25/.75) to get whole numbers. If percentages don't exactly add to 100% due to rounding errors in the original data, the algorithm still produces the correct empirical formula for most common compounds.
FAQ
What is an empirical formula?
An empirical formula is the simplest whole-number ratio of atoms in a compound. For example, glucose (C6H12O6) has empirical formula CH2O. It shows the relative number of each element but not the actual number of atoms in a molecule.
Is this calculator free?
Yes, completely free with no signup or account required. All calculations run in your browser.
Is my data private?
Yes. All calculations run locally in your browser. No data is sent to any server.
How do I find the molecular formula from the empirical formula?
Enter the molecular weight (molar mass) of the compound. The calculator divides it by the molar mass of the empirical formula to find the multiplier n, then multiplies all subscripts by n. For CH2O with MW=180 g/mol, n=180/30=6, giving molecular formula C6H12O6.
What if the mole ratios don't come out as whole numbers?
The calculator handles common non-integer ratios. A ratio ending in .5 is multiplied by 2; one ending in .33 or .67 is multiplied by 3; one ending in .25 or .75 is multiplied by 4. Values within 0.1 of a whole number are rounded directly.
Why do mass percentages need to add up to 100%?
Mass percent represents the fraction of each element in a 100g sample. If percentages don't sum to 100%, a common case is oxygen (not listed), which the calculator handles by computing the remaining percentage as oxygen.
What elements are available?
The calculator includes all common elements used in general chemistry courses — H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Fe, Cu, Zn, Br, Ag, I, Au, Pb, Hg, and others. You can type any element symbol.