The mole is the chemist’s counting unit. Reactions happen between whole numbers of particles – one molecule of methane burns with two molecules of oxygen – but in the lab we measure grams. The mole links the two: one mole of any substance contains 6.022 × 10²³ particles and has a mass in grams equal to its molar mass. So 18.015 g of water, 58.44 g of table salt and 180.16 g of glucose are each exactly one mole.
Three quantities belong together: mass m, amount n and molar mass M, related by n = m/M. Know any two and you have the third. The molar mass comes from the chemical formula, so this calculator reads the formula for you – type NaCl, H2SO4 or CuSO4·5H2O and it adds up the atomic weights. If the substance is unknown, enter a mass and a number of moles and the calculator gives the molar mass instead.
As a bonus the results include the number of molecules (or formula units) and, when you give a formula, the total number of atoms. These are the numbers you need to start any stoichiometry problem: convert grams to moles, use the mole ratio of the balanced equation, and convert back.
How to use the Mole Calculator
- Calculate
- Molar mass from
- Chemical formula
- Molar mass
- Mass
- Amount of substance
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
With a formula, the molar mass is the sum of the IUPAC standard atomic weights. The calculator then divides mass by molar mass (moles), multiplies moles by molar mass (mass) or divides mass by moles (molar mass), and multiplies the moles by Avogadro’s number to count particles.
Worked example
How many moles are in 10 g of table salt? M(NaCl) = 22.990 + 35.45 = 58.44 g/mol, so n = 10 ÷ 58.44 = 0.1711 mol. That is 0.1711 × 6.022 × 10²³ = 1.030 × 10²³ formula units, containing 2.061 × 10²³ ions (Na⁺ and Cl⁻).
Mass from moles: 0.25 mol of sulfuric acid (98.072 g/mol) weighs 0.25 × 98.072 = 24.52 g.
Solving a stoichiometry problem with moles
1. Convert the given mass to moles with n = m/M. 2. Use the coefficients of the balanced equation to get moles of the wanted substance. 3. Convert those moles back to mass with m = n × M. Example: burning 16.04 g of methane (1.000 mol) in CH₄ + 2 O₂ → CO₂ + 2 H₂O produces 1.000 mol of CO₂, which is 44.01 g.
Tips
- Always start from a balanced equation – coefficients give the mole ratio.
- Convert everything to moles first, then use the ratio, then convert back.
- Every field has its own unit menu – g, mg, mol, mmol, L and mL all work.
- Everything is calculated in your browser.
Common mistakes to avoid
- Using mass ratios instead of mole ratios.
- Forgetting water of crystallisation in the molar mass of hydrates.
- Confusing molarity (per litre of solution) with molality (per kilogram of solvent).
Formula
n = m ÷ M · m = n × M · M = m ÷ n · number of molecules = n × 6.02214076 × 10²³Worked example
Inputs — Calculate: Moles from mass; Molar mass from: Chemical formula; Chemical formula: NaCl; Mass: 10 g.
Results — Amount of substance = 0.171116; Mass = 10; Molar mass = 58.44; Number of molecules / formula units = 1.0305 × 10²³; Number of atoms = 2.061 × 10²³ atoms (2 per formula unit); Formula = NaCl.
Key terms
- Amount of substance (n)
- The number of entities expressed in moles.
- Molar mass (M)
- Mass of one mole, in g/mol; from the formula as the sum of atomic weights.
- Stoichiometry
- The quantitative relationships between reactants and products in a chemical reaction.
Frequently asked questions
How do I convert grams to moles?
Divide the mass by the molar mass: 10 g of NaCl ÷ 58.44 g/mol = 0.1711 mol.
How do I convert moles to grams?
Multiply by the molar mass: 0.25 mol of H₂SO₄ × 98.07 g/mol = 24.52 g.
How do I find molar mass from mass and moles?
Divide the mass by the moles. If 4.0 g of a gas is 0.125 mol, M = 32 g/mol (oxygen, O₂).
How many moles are in 100 g of water?
100 ÷ 18.015 = 5.551 mol, or 3.343 × 10²⁴ molecules.
How much NaOH neutralises 10 g of HCl?
10 g ÷ 36.46 g/mol = 0.2743 mol HCl; the 1 : 1 reaction needs 0.2743 mol NaOH × 40.00 g/mol = 10.97 g.
What is the formula for moles?
n = m/M, where m is the mass in grams and M the molar mass in g/mol.
Why use moles instead of grams in equations?
Coefficients in a balanced equation are ratios of particles, and moles count particles; equal masses of different substances contain different numbers of particles.
What formula does the Mole Calculator use?
n = m ÷ M · m = n × M · M = m ÷ n · number of molecules = n × 6.02214076 × 10²³.