Molecular Weight Calculator

Type any chemical formula, such as H2O, C6H12O6, Ca(OH)2 or CuSO4·5H2O, and get its molecular weight in grams per mole, a breakdown of each element’s contribution and mass percent, and a quick grams ↔ moles ↔ molecules converter. Standard atomic weights are built in.

What molecular weight means

The molecular weight of a substance is the sum of the atomic weights of all the atoms in its formula. Expressed in grams per mole (g/mol), it is also called the molar mass: the mass of one mole — 6.022 × 10²³ molecules or formula units — of that substance. Water, H₂O, has a molecular weight of 2 × 1.008 + 15.999 = 18.015 g/mol, so 18.015 grams of water contain one mole of water molecules. For ionic compounds such as sodium chloride, which do not form discrete molecules, the same sum is called the formula weight.

How to calculate molecular weight by hand

1. Write the formula and count the atoms of each element, multiplying through any brackets.
2. Look up each element’s standard atomic weight on the periodic table.
3. Multiply each atomic weight by its number of atoms.
4. Add the results.

Example — calcium hydroxide, Ca(OH)₂: Ca 40.078 + O 2 × 15.999 + H 2 × 1.008 = 74.092 g/mol.

Formula syntax the calculator understands

Capitalisation matters: “Co” is cobalt, but “CO” is carbon monoxide.

Mass percent composition

The table shows how much each element contributes to the total mass. In copper(II) sulfate pentahydrate, CuSO₄·5H₂O (249.68 g/mol), copper makes up about 25.45% of the mass, and the five water molecules together account for about 36% — which is why the blue crystals lose so much mass when heated to drive off the water. Percent composition is used to check purity, work out empirical formulas from lab data and calculate how much of an element a fertiliser or supplement supplies.

Grams, moles and molecules

Enter a mass to see the moles and number of formula units. For example, 10 g of CuSO₄·5H₂O is 0.04005 mol, about 2.41 × 10²² formula units.

Worked examples

Small differences in the last decimal place between sources come from which atomic weight values are used.

Atomic weights used

The calculator uses IUPAC abridged standard atomic weights, rounded to a practical number of digits. For elements with no stable isotopes, such as technetium and the heaviest elements, it uses the mass number of the most stable or best-known isotope, shown in brackets on many periodic tables. Some atomic weights vary slightly in nature (for example hydrogen, carbon and oxygen), which is why tables sometimes show intervals; the conventional values used here are appropriate for coursework and general laboratory calculations.

Uses

• Preparing solutions of a given molarity in the lab.
• Stoichiometry: converting between masses of reactants and products.
• Checking homework and worked examples.
• Calculating the percentage of an active ingredient or nutrient.
• Finding empirical formulas from percent composition.

Empirical and molecular formulas

The molecular formula gives the actual number of atoms in a molecule (glucose, C₆H₁₂O₆), while the empirical formula gives the simplest whole-number ratio (CH₂O). Molecular weight is calculated from the molecular formula. To find a molecular formula from an empirical one, divide the measured molar mass by the empirical formula mass and multiply the subscripts by the result: glucose’s molar mass 180.16 ÷ 30.03 (CH₂O) = 6, so C₆H₁₂O₆.

Molecular weight in the lab

Laboratory recipes usually specify concentrations in molarity (moles per litre). To make 250 mL of 0.5 M sodium chloride solution, you need 0.5 × 0.250 = 0.125 mol, which is 0.125 × 58.44 = 7.31 g of NaCl. For hydrated salts, use the molar mass of the hydrate you are weighing: 0.1 mol of copper sulfate pentahydrate is 24.97 g, whereas 0.1 mol of anhydrous copper sulfate is 15.96 g. Mixing these up is one of the most common errors in preparing solutions.

Common mistakes

• Writing element symbols in the wrong case (CO for cobalt).
• Forgetting to multiply everything inside brackets by the subscript outside.
• Leaving out water of crystallisation for hydrates.
• Using mass numbers (whole numbers) instead of standard atomic weights.
• Rounding too early in multi-step calculations.

Privacy

All calculations run in your browser; formulas you type are not stored or sent anywhere.