Molecular Formula Calculator

Determine the molecular formula from elemental composition data (mass % or grams) and molar mass. For students, researchers, and lab technicians. All calculations run instantly and offline in your browser.

Educational only. Follow your lab SOPs and SDS.

Input Data

Mass in grams

Percentage composition

Element	Mass (g) / %	Atomic Mass (g/mol)

Known Molar Mass (Optional, for molecular formula)

g/mol

Leave empty to calculate empirical formula only.

Decimal places:

Results

Empirical Formula

Lowest whole‑number ratio of atoms.

Molecular Formula

Actual formula based on molar mass.

Calculation Details

Element	Moles	Mole Ratio	Nearest Integer

How to Determine Molecular Formulas

A molecular formula specifies the exact number of each type of atom in a molecule. This is critical for stoichiometry, reaction planning, and verifying synthesis products.

Step‑by‑Step Usage

Add rows for each element in your compound.
Enter the experimental mass (in grams) or mass percentage for each element.
Provide the atomic mass (defaults are pre‑filled for common elements).
If the compound's molar mass is known, enter it to get the molecular formula.
Click "Calculate". Review the empirical formula and the multiplier to molecular formula.

Example Calculations

Scenario	Input	Result (Empirical)	Molar Mass	Molecular Formula
Glucose	C: 40.00%, H: 6.71%, O: 53.29%	CH₂O	180.16 g/mol	C₆H₁₂O₆
Aspirin	C: 4.35 g, H: 0.36 g, O: 1.73 g	C₉H₈O₄	180.16 g/mol	C₉H₈O₄
Hydrate	Cu: 3.97 g, S: 2.00 g, O: 4.02 g, H₂O: 2.25 g	CuSO₄·5H₂O	249.68 g/mol	CuSO₄·5H₂O

Common Mistakes Checklist

Using atomic mass incorrectly (e.g., Cl = 35 instead of 35.45).
Forgetting to convert percentages to grams (assume 100 g total).
Rounding intermediate mole ratios too early.
Confusing empirical and molecular formulas.
Not accounting for waters of hydration separately.

Quick Atomic Mass Reference

C: 12.011

H: 1.008

O: 15.999

N: 14.007

S: 32.06

Cl: 35.45

Click an element to insert its atomic mass into the active input above.

Frequently Asked Questions

What if my experimental percentages don't add up to 100%?

Small deviations are normal due to measurement error. The calculator normalizes to 100% automatically when in percentage mode. For best results, ensure your total is within 98‑102%.

How does it handle rounding to get integer subscripts?

The tool computes mole ratios, divides by the smallest, then rounds to the nearest integer if within 0.1 tolerance. If a ratio is far from an integer (e.g., 1.33), it multiplies all ratios by an integer factor (2, 3, etc.) until all are within tolerance.

Can I analyze hydrates or compounds with bound water?

Yes. Add "H" and "O" separately, or add a row for "H2O" with its molecular mass (18.015 g/mol) and treat it as a single component. Ensure your molar mass includes the water molecules.

What's the difference between empirical and molecular formula?

Empirical is the simplest whole‑number ratio (e.g., CH₂O). Molecular is the actual number of atoms in the molecule, which is a multiple of the empirical formula (e.g., C₆H₁₂O₆). The multiplier = molar mass / empirical formula mass.

Are there safety considerations?

This tool only performs calculations. Always verify chemical identities with analytical methods (NMR, mass spec). Incorrect formulas can lead to dangerous stoichiometric errors in synthesis.

Formulas & Assumptions

Moles of element: n_i = mass_i / atomic mass_i
Mole ratio: divide all n_i by the smallest n_i
Integer conversion: multiply ratios by smallest integer that makes all within ±0.1 of whole numbers
Empirical formula mass: sum of (atomic mass × subscript) for each element
Multiplier: k = round( molar mass / empirical formula mass )
Molecular formula subscripts: empirical subscript × k
Constants: Atomic masses from IUPAC 2019 standard values (common elements).
Rounding: Final subscripts are integers; intermediate values use selected decimal places.

Disclaimer: For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. This tool does not replace professional judgment, lab SOPs, or safety procedures. Always follow SDS guidance and institutional policies.

Sources & References: Common chemistry constants: R, N_A, F; IUPAC atomic weights; Standard unit conversions and lab measurement best practices; General analytical chemistry and uncertainty concepts.

Menu

Molecular Formula Calculator

AI Assistant

Was this tool helpful?