Mole Calculator: Convert Moles to Mass

Mole Calculator

A mole calculator converts between the amount of a substance in moles and its mass in grams, using the substance's molar mass. It works in both directions: moles to mass, and mass to moles.

What is a mole?

The mole (symbol: mol) is the SI base unit for the amount of a substance. One mole of any substance contains exactly 6.02214076 × 10²³ elementary particles, such as atoms, molecules, or ions. This fixed number is called Avogadro's number, or Avogadro's constant.

The mole gives chemists a way to count particles by weighing them. Nobody can count individual atoms on a scale, but a scale can weigh grams. The mole links these two scales together.

Before 2019, the mole was defined using carbon-12: one mole was the amount of a substance containing as many entities as there are atoms in exactly 12 grams of carbon-12. In 2019, the International Bureau of Weights and Measures (BIPM) redefined the mole by fixing Avogadro's number at exactly 6.02214076 × 10²³, rather than tying it to a physical sample.

Mole to mass formula

Two formulas connect moles, mass, and molar mass:

Mass (g) = Moles (mol) × Molar mass (g/mol)

Moles (mol) = Mass (g) ÷ Molar mass (g/mol)

Molar mass, also called molecular weight, is the mass of one mole of a substance. It is measured in grams per mole (g/mol) and equals the sum of the atomic weights of every atom in the substance's chemical formula.

How to calculate mass from moles

To find mass, multiply the number of moles by the molar mass.

Example: mass of 2 moles of water

Water (H₂O) has a molar mass of 18.015 g/mol.

Mass = 2 mol × 18.015 g/mol = 36.03 g

Two moles of water weigh 36.03 grams.

How to calculate moles from mass

To find moles, divide the mass by the molar mass.

Example: moles in 100 grams of table salt

Sodium chloride (NaCl) has a molar mass of 58.44 g/mol.

Moles = 100 g ÷ 58.44 g/mol = 1.71 mol

100 grams of table salt contains about 1.71 moles of NaCl.

How to find molar mass

Molar mass is found by adding up the atomic weights of every atom in a formula, using values from a periodic table.

For water (H₂O):

• 2 hydrogen atoms: 2 × 1.008 = 2.016 g/mol
• 1 oxygen atom: 1 × 16.00 = 16.00 g/mol
• Total: about 18.02 g/mol, commonly rounded to 18.015

For compounds that include water molecules bound in their crystal structure, called hydrates, the water must be added too. Copper sulfate pentahydrate (CuSO₄·5H₂O) has a molar mass of about 249.68 g/mol: 159.61 g/mol for the anhydrous CuSO₄, plus 5 × 18.015 g/mol for the five water molecules.

Common molar masses

Why the mole matters in chemistry

Chemical formulas describe ratios of particles, but lab equipment measures mass. The mole converts between the two, so it appears throughout chemistry.

In stoichiometry, a reaction's balanced equation gives mole ratios between reactants and products. To predict how much product a reaction will make, a chemist converts the starting mass to moles, applies the mole ratio from the equation, then converts back to mass.

In solution preparation, molarity describes concentration as moles of solute per litre of solution. To prepare 500 mL of a 0.1 mol/L sodium hydroxide solution, the required amount of substance is 0.1 mol/L × 0.5 L = 0.05 mol. Converting to mass gives 0.05 mol × 40.00 g/mol = 2.00 g of NaOH.

Frequently asked questions

How do you convert moles to grams?

Multiply the number of moles by the molar mass: mass (g) = moles (mol) × molar mass (g/mol). For 2 moles of water, that is 2 × 18.015 = 36.03 g.

How do you convert grams to moles?

Divide the mass by the molar mass: moles (mol) = mass (g) ÷ molar mass (g/mol). For 100 g of NaCl, that is 100 ÷ 58.44 ≈ 1.71 mol.

What is Avogadro's number?

Avogadro's number is 6.02214076 × 10²³, the exact number of particles in one mole of any substance. It is named after the Italian scientist Amedeo Avogadro, who in 1811 proposed that equal volumes of gas at the same temperature and pressure contain equal numbers of molecules.

What is the difference between molecular weight and molar mass?

In everyday lab use, the two terms mean the same thing and share the same value in g/mol. Strictly, molecular weight is a ratio with no units, comparing a molecule's mass to one-twelfth the mass of a carbon-12 atom, while molar mass carries units of g/mol. Textbooks and calculators generally use the terms interchangeably.

How many grams are in one mole?

It depends on the substance. One mole of any substance weighs exactly its molar mass in grams: one mole of carbon weighs 12.01 g, one mole of water weighs 18.015 g, and one mole of glucose weighs 180.16 g.

Can this calculator be used to check chemistry homework?

Yes. It applies the standard mass–mole formulas, so it can verify a hand calculation or a homework answer. Working the calculation by hand first is still useful for understanding stoichiometry and significant figures.

References

1. Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), 9th edition, 2019.
2. National Institute of Standards and Technology (NIST). NIST Chemistry WebBook. https://webbook.nist.gov/chemistry/
3. International Union of Pure and Applied Chemistry (IUPAC). Compendium of Chemical Terminology (Gold Book). https://goldbook.iupac.org/