Lesson 6 of 7 · Chemical calculations
Molar mass
Molar mass is the mass of one mole of a substance, usually given in grams per mole (g/mol). To calculate it from a formula, multiply each element’s atomic-weight value by the number of its atoms, then add the results. The total gives the numerical molar mass in g/mol at classroom precision.
Molar mass calculator
Enter a neutral chemical formula such as Ca(OH)2 or CuSO4·5H2O, or pick an example below. Element symbols are case-sensitive.
Molar mass of H2O
18.015 g/mol
3 atoms across 2 elements
| Element | Atoms | Atomic weight | g/mol | Mass % |
|---|---|---|---|---|
| H | 2 | 1.008 | 2.016 | 11.19% |
| O | 1 | 15.999 | 15.999 | 88.81% |
Supported elements and atomic weights
Conventional classroom values in the calculation: H 1.008; He 4.0026; Li 6.94; Be 9.0122; B 10.81; C 12.011; N 14.007; O 15.999; F 18.998; Ne 20.18; Na 22.99; Mg 24.305; Al 26.982; Si 28.085; P 30.974; S 32.06; Cl 35.45; Ar 39.95; K 39.098; Ca 40.078; Sc 44.956; Ti 47.867; V 50.942; Cr 51.996; Mn 54.938; Fe 55.845; Co 58.933; Ni 58.693; Cu 63.546; Zn 65.38; Br 79.904; Ag 107.87; I 126.9; Ba 137.33; Pt 195.08; Au 196.97.
Based on CIAAW abridged atomic weights. These values are for ordinary classroom calculations. The tool does not calculate isotope-specific masses or uncertainties.
Count before calculating
A subscript applies to the symbol or bracketed group immediately before it. Ca(OH)₂ contains one Ca, two O and two H atoms per formula unit. In CuSO₄·5H₂O, five water molecules accompany each CuSO₄ formula unit: count Cu₁S₁O₉H₁₀ overall.
Molar mass has units of g/mol. Relative molecular mass is dimensionless; molecular mass is often given in u or Da. These quantities share closely corresponding numerical values at classroom precision but describe different things. Ionic solids such as NaCl are discussed in terms of formula units, not discrete NaCl molecules.
Reference: OpenStax Chemistry 2e, §3.1: Formula mass and the mole
Calculate mass percentages
Mass percent of an element = (that element’s total mass contribution ÷ formula molar mass) × 100%. Divide by the complete formula mass, including every subscript and any waters of crystallization.
Mass percentage is not atomic percentage. In H₂O, hydrogen accounts for two of the three atoms but only about 11.19% of the mass with the values used here. Keep unrounded intermediate values, then round the reported result.
| Element | Count | Atomic weight | Contribution (g/mol) |
|---|---|---|---|
| H | 2 | 1.008 | 2.016 |
| O | 1 | 15.999 | 15.999 |
| Total | 18.015 |
Reference: OpenStax Chemistry 2e, §3.2: Composition and formulas
Understand the precision
Samples of the same element can contain slightly different proportions of isotopes. That is why some standard atomic weights are given as intervals. A single conventional value is useful for homework, but a precise laboratory calculation may need the isotopic composition of the actual sample.
The calculator works with the listed common elements and neutral formulas. You can use parentheses, brackets and the middle dot · for hydrates. Isotope labels, ionic charges, reaction coefficients and fractional atom counts are not supported. Check the atomic weights shown with the result and round your answer to the precision required by your course.
Reference: CIAAW: Abridged standard atomic weights
Work through the examples
Worked example 1
Find the molar mass of Ca(OH)₂.
- Expand the group: Ca₁O₂H₂.
- Compute 40.078 + 2(15.999) + 2(1.008).
Answer: 74.092 g/mol using the listed atomic weights.
Worked example 2
Find hydrogen’s mass percentage in H₂O.
- Hydrogen contributes 2 × 1.008 = 2.016 g/mol.
- Divide by 18.015 g/mol and multiply by 100.
Answer: About 11.19% hydrogen by mass.
Avoid these mistakes
- Do not multiply only the last atom inside parentheses.
- A hydrate dot is not a decimal point.
- Do not substitute a radioactive isotope’s mass number for a standard atomic weight.
Check your understanding
Open printable worksheetTry each question before opening the answer key. When you check your answers, read the explanations too. Your choices stay in this browser.
Read the complete answer key
1. C — 6. The outside subscript multiplies every atom in NO₃: 3 × 2 = 6 oxygen atoms.
2. A — g/mol. Molar mass is mass divided by amount of substance.
3. B — Element mass contribution ÷ total formula mass. Multiply this mass fraction by 100 to obtain a percentage.
4. C — 10. Five waters each contribute two hydrogen atoms, giving ten.
Common questions
Why do two calculators give slightly different answers?
They may use differently rounded atomic weights or different conventions for elements with variable isotopic composition. Compare the underlying values and requested precision.
Can this calculate the mass of an isotope-labeled compound?
No. Isotope-specific calculations require isotope masses and a different input notation. This tool uses conventional atomic-weight values for ordinary classroom formulas.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.