The Molar Mass Calculator computes molecular weight from a chemical formula and converts between chemistry units.
Supports automatic chemical formula parsing including parentheses (e.g., Ca(OH)2). Converts between grams, moles, and particle count using Avogadro's number.
Calculator information
📋 How to use this calculator
- Enter the chemical formula with correct capitalization (e.g., H2O, NaCl, Ca(OH)2).
- Use parentheses for repeating groups, such as Al2(SO4)3 or (NH4)2CO3.
- Click Calculate to get the molar mass in g/mol along with each element's contribution.
- Choose conversion mode: grams to moles, moles to grams, or moles to number of particles.
- Enter the value to convert in the appropriate input field with clear units.
- Tip: for hydrates such as CuSO4·5H2O, use a middle dot separator so all components are counted.
🧮 Mass-Mole-Particle Conversion
n = m / M; m = n × M; N = n × N_A
- n = number of moles (mol)
- m = mass (grams)
- M = molar mass (g/mol) = Σ Ar × number of atoms
- N = number of particles (atoms, molecules, ions)
- N_A = Avogadro's number = 6.022 × 10²³ particles/mol
- Ar = relative atomic mass (see IUPAC periodic table)
Since 2019, IUPAC defines the mole exactly as 6.02214076 × 10²³ elementary entities.
💡 Worked example: Find the molar mass of Ca(OH)2 and convert 10 g to moles
Given:- Formula = Ca(OH)2
- Ar Ca = 40.08; Ar O = 16.00; Ar H = 1.008
- Given mass = 10 grams
Steps:- Count atoms: 1 Ca, 2 O, 2 H.
- Ca contribution = 1 × 40.08 = 40.08 g/mol.
- O contribution = 2 × 16.00 = 32.00 g/mol.
- H contribution = 2 × 1.008 = 2.016 g/mol.
- M(Ca(OH)2) = 40.08 + 32.00 + 2.016 = 74.096 g/mol; n = 10 / 74.096 ≈ 0.1350 mol.
Result: M = 74.10 g/mol; 10 grams of Ca(OH)2 equals 0.135 mol or ≈ 8.13 × 10²² formula units.
❓ Frequently asked questions
What is a mole and why does it matter?
The mole is the SI unit for amount of substance, equal to Avogadro's number (6.022 × 10²³) of elementary particles. The mole connects the microscopic world (atoms, molecules) with the macroscopic (grams), allowing chemical reactions to be quantified. Without the mole, reaction stoichiometry would be impractical.
What's the difference between relative atomic mass (Ar) and relative molecular mass (Mr)?
Ar is the average mass of one atom of an element relative to 1/12 of the mass of carbon-12. Mr is the sum of all Ar values in a molecule. Molar mass (M) is numerically equal to Mr but has units of g/mol. Updated Ar values are published regularly by IUPAC.
How do I calculate the molar mass of a hydrate?
Hydrates are written with a middle dot, e.g., CuSO4·5H2O. Calculate the molar mass of the anhydrous part, then add 5 times the molar mass of H2O (5 × 18.015 = 90.075 g/mol). Total M(CuSO4·5H2O) ≈ 159.61 + 90.08 = 249.69 g/mol.
Is Avogadro's number the same for every substance?
Yes. Avogadro's number is a universal constant independent of the substance. 1 mol of H2O, 1 mol of Fe, or 1 mol of electrons all contain 6.022 × 10²³ particles. Only the mass per mole differs because each substance has a different molar mass.
How is molar mass related to gas volume?
At IUPAC STP (0°C, 100 kPa), 1 mol of an ideal gas occupies 22.711 L. For other conditions, use the ideal gas law PV = nRT. The molar mass of a gas (M = mRT/PV) can be determined by lab measurements of pressure, temperature, volume, and mass.
📚 Sources & references
Last updated: May 11, 2026