Moles, grams, and molar mass
The mole is the SI unit for amount of substance. Molar mass links that amount to sample mass by stating how many grams correspond to one mole of a substance.
This calculator solves the mass–mole relationship. It does not identify a compound or calculate its molar mass from a formula; use the molecular-weight calculator for that step.
How to use the mole calculator
- Choose the unknown: Select moles, sample mass, or molar mass.
- Use the exact substance: Enter molar mass for the correct formula, hydrate, isotope convention, or mixture definition.
- Match units: Use grams and g/mol to obtain moles directly.
- Calculate: Review whether the displayed precision is justified by the inputs.
Formula and variables
Use mass m in grams and molar mass M in g/mol to obtain amount n in moles.
n = m/M; m = nM; M = m/n- n — Amount of substance
- Chemical amount represented by the sample (mol)
- m — Sample mass
- Mass of the substance (g)
- M — Molar mass
- Mass per mole of the substance (g/mol)
Water sample
A water sample has mass 36.03 g and molar mass 18.015 g/mol.
- Mass
- 36.03 g
- Molar mass
- 18.015 g/mol
- n = 36.03/18.015
Result: The sample amount is 2 mol.
The sample contains two moles of H₂O formula entities under the stated molar mass.
Understanding your results
Moles measure amount, not mass
Equal mole amounts of different substances contain equal numbers of specified entities but usually have different masses.
- Zero mass corresponds to zero moles for a positive molar mass.
- Molar mass must refer to the same substance as the sample.
- For ionic compounds, the counted entities are formula units rather than discrete molecules.
- Significant figures should follow the least precise measured input.
Assumptions
- Sample composition and molar mass are known and uniform.
- Mass and molar mass use compatible gram-based units.
- The entered quantities describe the same chemical entity.
Limitations
- Does not calculate particles directly or parse chemical formulas.
- Does not correct for purity, solvent, hydration, mixtures, or measurement uncertainty.
- Average molar mass may not represent an isotopically enriched material.
Common mistakes
- Multiplying mass by molar mass when solving for moles.
- Using kilograms with a g/mol molar mass without conversion.
- Confusing molecules with formula units.
- Using the molar mass of the wrong hydrate or chemical form.
Practical use cases
Stoichiometry setup
Convert measured reagent mass into moles before applying reaction coefficients.
Experimental molar mass
Calculate mass divided by an independently measured positive mole amount.
Frequently asked questions
How do I convert grams to moles?
Divide mass in grams by molar mass in grams per mole.
How do I convert moles to grams?
Multiply moles by molar mass in grams per mole.
Does one mole always have the same mass?
No. One mole contains the same number of specified entities, but mass depends on the substance’s molar mass.
Sources and review
- Formula Mass and the Mole Concept — OpenStax Chemistry 2e. Accessed 2026-07-13.
Reviewed 2026-07-13.