Grams to Moles Calculator
Enter the mass in grams and the molar mass (g/mol). You can also pick a common compound to auto-fill molar mass.
How to convert grams to moles
Converting grams to moles is one of the most useful skills in chemistry. Mass is what we can directly measure on a balance, but chemical reactions are governed by amount of substance, which we express in moles. This conversion links the real world (grams) to chemical equations (moles).
moles = grams ÷ molar mass
where molar mass is in g/mol.
Step-by-step method
- Step 1: Find the mass in grams (given or measured).
- Step 2: Determine the molar mass of the compound (g/mol).
- Step 3: Divide grams by molar mass.
- Step 4: Report the answer in moles (mol), using appropriate significant figures.
Example 1: Water
If you have 36.03 g of water (H₂O), and molar mass is 18.015 g/mol:
moles = 36.03 ÷ 18.015 = 2.000 mol
Example 2: Sodium chloride
For 5.844 g NaCl, with molar mass 58.44 g/mol:
moles = 5.844 ÷ 58.44 = 0.1000 mol
Common molar masses (quick reference)
| Compound | Formula | Molar Mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Carbon dioxide | CO₂ | 44.009 |
| Sodium chloride | NaCl | 58.44 |
| Glucose | C₆H₁₂O₆ | 180.156 |
| Ammonia | NH₃ | 17.031 |
How to find molar mass manually
If your compound is not in the list, calculate molar mass by summing the atomic masses from the periodic table:
- Write the chemical formula.
- Count each atom in the formula.
- Multiply each atom count by its atomic mass.
- Add all contributions together.
Example: For CO₂: 1×C (12.01) + 2×O (16.00) = 44.01 g/mol.
Common mistakes to avoid
- Using the wrong molar mass (especially for hydrates and polyatomic compounds).
- Forgetting units. Grams and g/mol must be consistent.
- Rounding too early. Keep extra digits until the final step.
- Confusing grams-to-moles with moles-to-grams (inverse operations).
Why this conversion matters
Grams-to-moles conversion appears in stoichiometry, limiting reagent problems, solution preparation, gas calculations, and lab yield analysis. Once you can move between grams and moles confidently, balancing and solving reaction problems becomes much easier.
FAQ
Can I use this for elements and compounds?
Yes. For elements, use the atomic molar mass (e.g., Fe = 55.845 g/mol). For compounds, use total formula mass (e.g., H₂SO₄ = 98.079 g/mol).
What if the calculator gives a very small number?
That is normal for tiny sample masses or very large molar masses. Scientific notation is often used in chemistry for these results.
Do I need perfect precision?
Use reasonable significant figures based on your measurements. In most class and lab settings, 3–4 significant figures is acceptable unless specified otherwise.