Molar Mass Calculator

Calculate a compound's molar mass from its chemical formula, see each element's contribution and percent composition, then convert grams and moles.

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How to calculate molar mass without losing an atom

Molar mass turns the atom counts in a chemical formula into the mass of one mole. The reliable method has three moves: count, multiply, add. For H2O, count two hydrogen atoms and one oxygen atom. Multiply 2 × 1.0080 for hydrogen and 1 × 15.999 for oxygen, then add 2.016 + 15.999. The result is 18.015 g/mol.

The arithmetic is usually the easy part. Most wrong answers begin one step earlier, when a subscript is applied to too few or too many atoms. Work from the innermost group outward and write the expanded atom count before touching a calculator.

  1. List each element once. Preserve the order in which it first appears so the work is easy to compare with the original formula.
  2. Expand every group. In Al2(SO4)3, the outside 3 gives three sulfur atoms and twelve oxygen atoms. Aluminium keeps its separate subscript of 2.
  3. Multiply count by atomic weight. The contribution column records this step for every element.
  4. Add contributions before rounding. Keeping the displayed precision until the end avoids a small drift across a formula with several elements.

A useful classroom routine is to have students submit the expanded counts first. Once Al2S3O12 is correct, the remaining calculation becomes a check of multiplication and addition rather than a second test of notation.

Parentheses, hydrates, and charges: where formulas go wrong

Three marks change how a formula is read: a closing-group subscript, a hydrate dot, and a trailing charge. They look small on the page, but each has a different job.

  • A subscript after a group multiplies the entire group. Ca(OH)2 contains Ca1O2H2. Nested groups follow the same rule from the inside outward, and matching (), [], or {} pairs keep the boundaries clear.
  • A hydrate coefficient multiplies the formula after the dot. CuSO4·5H2O adds five water units to copper sulfate. That contributes H10O5 on top of the O4 already present, for Cu1S1O9H10.
  • A charge changes electron count, not the written atom count. NH4+ still contains one nitrogen and four hydrogen atoms. For SO4^2−, the caret makes it clear that 2 is the charge magnitude rather than part of oxygen's subscript.

Capitalization is chemistry, not typography. Co is cobalt, while CO contains carbon and oxygen. A lowercase first letter is rejected because silently changing it could turn one substance into another. State labels such as (s), (l), (g), and (aq) are not part of the composition and should be left out.

Turn the breakdown into percent composition

Atom count and mass share are not the same thing. Water has two hydrogen atoms for every oxygen atom, yet oxygen supplies most of the mass because one oxygen atom is almost sixteen times as heavy as one hydrogen atom. The breakdown makes that contrast visible.

Use the same rule for every row: element contribution ÷ total molar mass × 100. For oxygen in H2O, 15.999 ÷ 18.015 × 100 = 88.81%. Hydrogen contributes the remaining 11.19%. The unrounded percentages add to exactly 100%; displayed rows may differ by a hundredth because each one is rounded separately.

  • Prediction first: ask which element will supply the largest mass share before showing the table. Formulas with many light hydrogen atoms are especially useful.
  • Hide one column: print the worked result, cover the Percent column, and have students calculate each percentage from the contribution and total.
  • Compare related compounds: calculate CO and CO2 side by side. Carbon falls from about 42.88% of CO to about 27.29% of CO2 because the second oxygen changes the denominator as well as the oxygen contribution.

Percent composition is a mass statement. It does not say how many atoms are present in a sample, and it does not replace a balanced chemical equation.

Use molar mass as the bridge between grams and moles

A balanced equation works in particles and moles, while a laboratory balance reads grams. Molar mass is the bridge. Divide grams by g/mol to reach moles; multiply moles by g/mol to return to grams. Writing the units beside every number shows which operation is correct because grams cancel in the first direction and moles cancel in the second.

For 36.030 g of water, the calculation is 36.030 g ÷ 18.015 g/mol = 2.0000 mol. In the other direction, 0.500 mol × 18.015 g/mol = 9.0075 g. The converter keeps the formula and amount together on the printable worked result, which makes it suitable as an answer sheet or a model for dimensional analysis.

  • Teach the formula before the shortcut. Have students write grams ÷ molar mass or moles × molar mass before entering the amount.
  • Round at the end. The tool shows extra digits so a teacher can apply the significant-figure rules required by the assignment.
  • Check the source value. The calculator uses IUPAC CIAAW Abridged Standard Atomic Weights 2024. For 34 radioactive elements that have no IUPAC standard atomic weight, a bracketed mass number is clearly marked and the result should be treated as isotope-based.
  • Keep student data private. Formulas, amounts, validation, and print preparation all stay in the browser.

A fast reasonableness check catches reversed operations: more grams than one molar mass should produce more than one mole, while half a mole should weigh half the molar mass.

Frequently Asked Questions

Common questions about the Molar Mass Calculator

Enter the formula with element symbols and whole-number subscripts, such as H2O or CaCO3. The calculator counts every atom, multiplies each count by that element's atomic weight, and adds the contributions. The large result is the compound's molar mass in grams per mole, while the table shows every step.

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