How to Find Molar Mass

By MeasureBeam Editorial Team·September 2, 2026·5 min read

Molar mass is the mass of one mole of a substance. It’s measured in grams per mole (g/mol).

To find it, add up the atomic masses of every atom in the chemical formula. Those values come straight from the periodic table.

The formula

molar mass = sum of (atomic mass × number of atoms) for each element in the formula

Worked example: water (H₂O)

Water has 2 hydrogen atoms and 1 oxygen atom.

Hydrogen: 1.008 g/mol × 2 atoms = 2.016
Oxygen: 16.00 g/mol × 1 atom = 16.00
Total: 2.016 + 16.00 = 18.02 g/mol

Worked example: table salt (NaCl)

Sodium (Na): 22.99 g/mol × 1 = 22.99
Chlorine (Cl): 35.45 g/mol × 1 = 35.45
Total: 58.44 g/mol

Worked example: glucose (C₆H₁₂O₆)

Carbon: 12.01 × 6 = 72.06
Hydrogen: 1.008 × 12 = 12.10
Oxygen: 16.00 × 6 = 96.00
Total: 180.16 g/mol

Worked example: calcium hydroxide, Ca(OH)₂

Calcium (Ca): 40.08 × 1 = 40.08
Oxygen (O): 16.00 × 2 = 32.00
Hydrogen (H): 1.008 × 2 = 2.016
Total: 74.10 g/mol

Worked example: sulfuric acid, H₂SO₄

Hydrogen: 1.008 × 2 = 2.016
Sulfur: 32.07 × 1 = 32.07
Oxygen: 16.00 × 4 = 64.00
Total: 98.09 g/mol

Worked example: ammonium nitrate, NH₄NO₃

This one trips people up. Nitrogen appears twice, in two different groups, and has to be counted both times.

Nitrogen (N): 14.01 × 2 = 28.02 (one N in NH₄, one N in NO₃)
Hydrogen (H): 1.008 × 4 = 4.032
Oxygen (O): 16.00 × 3 = 48.00
Total: 80.05 g/mol

Reference table: common atomic masses

Element	Symbol	Atomic mass (g/mol)
Hydrogen	H	1.008
Carbon	C	12.01
Nitrogen	N	14.01
Oxygen	O	16.00
Sodium	Na	22.99
Sulfur	S	32.07
Chlorine	Cl	35.45
Calcium	Ca	40.08

Common molar masses for reference

Substance	Formula	Molar mass
Water	H₂O	18.02 g/mol
Carbon dioxide	CO₂	44.01 g/mol
Table salt	NaCl	58.44 g/mol
Glucose	C₆H₁₂O₆	180.16 g/mol
Oxygen gas	O₂	32.00 g/mol
Ethanol	C₂H₅OH	46.07 g/mol
Calcium hydroxide	Ca(OH)₂	74.10 g/mol
Sulfuric acid	H₂SO₄	98.09 g/mol
Ammonium nitrate	NH₄NO₃	80.05 g/mol
Methane	CH₄	16.04 g/mol
Sodium bicarbonate	NaHCO₃	84.01 g/mol

Step-by-step process

Write out the chemical formula exactly as given. Note every subscript.
List each distinct element in the formula separately.
Look up each element’s atomic mass. Use the number under the element symbol, not the atomic number above it.
Multiply each atomic mass by its subscript. That subscript is the number of atoms of that element in the formula. No visible subscript means exactly 1 atom.
Add every element’s total together. The sum is the molar mass, in grams per mole.
Check the parentheses. If a group sits inside parentheses with a subscript outside, every element inside gets multiplied by that outside number too. That’s on top of its own internal subscript.

This process works the same way for a simple diatomic gas. It also works for a complex organic compound.

Using molar mass to convert grams and moles

Once you have the molar mass, it works as a conversion factor. Moles = grams ÷ molar mass. Grams = moles × molar mass.

Say you have 90.08 grams of glucose. Molar mass is 180.16 g/mol. That’s 90.08 ÷ 180.16 = 0.5 moles.

This step is what makes molar mass useful in practice. Reactions happen mole-to-mole, according to a balanced equation. But you weigh out reagents in grams. Molar mass bridges the two.

Where the atomic mass numbers come from

Every element’s atomic mass sits on the periodic table, usually below its symbol. It’s a weighted average of that element’s naturally occurring isotopes.

That’s why the numbers — 1.008, 16.00, 35.45 — aren’t clean whole numbers. You don’t derive these yourself. You look them up and plug them into the formula above.

Common mistakes when you calculate molar mass

Forgetting to multiply by the subscript. In H₂O, the “2” after H means 2 hydrogen atoms. It’s easy to add hydrogen’s atomic mass only once instead of twice. That turns 18.02 g/mol into a wrong answer of 17.01 g/mol.

Confusing molar mass with molecular weight in a way that changes the setup. The numbers match for practical work. But molecular weight is a ratio with no units, while molar mass carries g/mol. Dropping the unit on a lab report can cost points even when the number itself is right.

Misreading a subscript outside parentheses. In Ca(OH)₂, a student might apply the 2 only to oxygen and forget hydrogen. That gives 40.08 + 32.00 + 1.008 = 73.09 g/mol. The correct answer, applying the 2 to both O and H, is 74.10 g/mol.

Skipping an element hidden in a second group. In NH₄NO₃, someone might count nitrogen only once, from NH₄, and miss the N in NO₃. That gives 66.04 g/mol instead of the correct 80.05 g/mol.

Frequently Asked Questions

What’s the difference between molar mass and molecular weight?
For most practical purposes, they’re the same number. “Molecular weight” is technically a dimensionless ratio, while “molar mass” carries the g/mol unit. In everyday chemistry, people use the terms interchangeably.

How do I find molar mass for a compound with parentheses, like Ca(OH)₂?
Multiply everything inside the parentheses by the subscript outside it first. Ca(OH)₂ has 1 calcium, 2 oxygen, and 2 hydrogen atoms. Work it out: 40.08 + (16.00 × 2) + (1.008 × 2) = 74.10 g/mol.

Why does molar mass matter outside a chemistry classroom?
It’s the conversion factor between mass and the number of particles in a sample. That matters for anything involving chemical reactions at a measurable scale — pharmaceutical dosing, industrial chemical mixing, even brewing.

What’s the difference between molar mass and molar volume?
Molar mass is the mass of one mole of a substance, in grams. Molar volume is the space one mole of a gas occupies, in liters. For any ideal gas at standard temperature and pressure, that’s about 22.4 L, regardless of which gas it is.

How do I determine the molar mass of an ion, like sulfate (SO₄²⁻)?
The same way as a neutral molecule. Add up the atomic masses of sulfur and 4 oxygens: 32.07 + (16.00 × 4) = 96.07 g/mol. Electrons weigh almost nothing compared to protons and neutrons, so the charge doesn’t meaningfully change the mass.

Can I work out molar mass without a periodic table in front of me?
Not accurately. Atomic masses come from measured isotope abundances, not something you can derive on the spot. Memorizing the common ones helps: H = 1.008, C = 12.01, O = 16.00, N = 14.01. That covers a lot of everyday organic chemistry. Anything beyond that needs a reference.

Why this calculation matters in real applications

Molar mass isn’t just a classroom exercise. A pharmacist calculating a drug dose uses it. So does an engineer designing a chemical process, and a student titrating an acid in a lab.

Each one converts between a measurable mass and the number of molecules that mass represents. Get the molar mass wrong by even a small formula error, like a missed subscript, and every downstream number inherits that error. A reaction yield, a medication dose, a batch specification — all of it.

That’s why the checking step matters as much as the formula. Recount the atoms in the formula, confirm each atomic mass against the periodic table, and re-add the total before trusting the final number.