The mole and molar mass
A mole is chemistry's counting unit — one mole is 6.022 × 10²³ particles (that's the Avogadro constant), just as a dozen is 12. Molar mass is the mass of one mole of a substance in grams per mole; you find it by adding up the relative atomic masses of every atom in the formula. The mole is what lets a chemist weigh out a substance and know how many particles they have — linking the balanced-equation world of ratios to grams on a balance.
Why chemists need the mole
Atoms are far too small and too numerous to count. A single drop of water holds around 1.7 × 10²¹ molecules. But reactions happen in simple whole-number ratios of atoms and molecules — a balanced equation might say "2 hydrogen molecules react with 1 oxygen molecule".
The mole bridges the two. It is a fixed, huge number of particles (6.022 × 10²³) chosen so that one mole of a substance weighs, in grams, the same number as its relative formula mass. So a chemist can weigh 18 g of water on a balance and know it contains exactly one mole — 6.022 × 10²³ molecules.
Relative atomic mass
Every element has a relative atomic mass (Ar) printed on the periodic table — hydrogen is about 1, carbon 12, oxygen 16, calcium 40. It's "relative" because it compares each atom's mass to one-twelfth of a carbon-12 atom, so the numbers have no units. The values aren't whole because most elements are a mix of isotopes (chlorine is 35.45 because it's roughly three-quarters chlorine-35 and one-quarter chlorine-37).
Calculating molar mass
The molar mass (M) of a compound is just the sum of the relative atomic masses of all its atoms, with units of g mol⁻¹.
- Write out each element in the formula and how many atoms of it there are (expand any brackets first).
- Multiply each count by that element's relative atomic mass.
- Add everything up.
Water, H₂O
| Element | Atoms | Ar | Contribution |
|---|---|---|---|
| H | 2 | 1.008 | 2.016 |
| O | 1 | 16.00 | 16.00 |
| Molar mass | 18.02 g mol⁻¹ | ||
Calcium carbonate, CaCO₃
| Element | Atoms | Ar | Contribution |
|---|---|---|---|
| Ca | 1 | 40.08 | 40.08 |
| C | 1 | 12.01 | 12.01 |
| O | 3 | 16.00 | 48.00 |
| Molar mass | 100.09 g mol⁻¹ | ||
Calcium nitrate, Ca(NO₃)₂ — watch the bracket
The subscript 2 multiplies everything inside the bracket: 2 N and 6 O.
| Element | Atoms | Ar | Contribution |
|---|---|---|---|
| Ca | 1 | 40.08 | 40.08 |
| N | 2 | 14.01 | 28.02 |
| O | 6 | 16.00 | 96.00 |
| Molar mass | 164.10 g mol⁻¹ | ||
Mass, moles and particles
Three quantities, two conversions. Cover the one you want to find:
Example. How many moles are in 25 g of calcium carbonate? moles = mass ÷ M = 25 ÷ 100.09 = 0.25 mol. And that's 0.25 × 6.022 × 10²³ ≈ 1.5 × 10²³ formula units. For a gas, one mole also occupies about 22.7 L at STP — see chemistry constants.
Practise — work it out, then check
Calculate the molar mass yourself, then enter it to check. If you're off, the working is shown so you can see where.
Try any formula
Type a formula, work out its molar mass, then enter your answer:
The mole — FAQ
What is a mole in chemistry?
An amount of substance containing exactly 6.02214076 × 10²³ particles. It's a counting unit — like a dozen, but sized so that one mole of a substance weighs its relative formula mass in grams.
What is molar mass?
The mass of one mole of a substance, in g mol⁻¹. For an element it equals the relative atomic mass; for a compound, add up the relative atomic masses of all the atoms in the formula.
How do you calculate molar mass?
List each element and its number of atoms (expand brackets), multiply each by its relative atomic mass, and add. CaCO₃ = 40.08 + 12.01 + 3×16.00 = 100.09 g mol⁻¹.
How do you convert grams to moles?
moles = mass ÷ molar mass. 50 g of water is 50 ÷ 18.02 = 2.77 mol.
How do you get the number of molecules?
Multiply moles by the Avogadro constant: particles = moles × 6.022 × 10²³.
Is molar mass the same as molecular mass?
The numbers are the same. "Molecular mass" (or relative formula mass) is unitless and refers to one molecule; "molar mass" carries the unit g mol⁻¹ and refers to a mole.