Molar mass calculator

Chemical formula

Capitals matter: Co is cobalt, but CO is carbon monoxide. Write hydrates with a dot (CuSO4·5H2O) and charges after a ^ (SO4^2-).

Examples:H₂ONaClH₂SO₄Ca(OH)₂CuSO₄·5H₂OC₆H₁₂O₆

Molar mass H₂SO₄

98.07 g/mol

Element	Atoms	Relative atomic mass	Subtotal (g/mol)	% by mass
H hydrogen	2	1.0080	2.0160	2.06
S sulfur	1	32.06	32.06	32.69
O oxygen	4	15.999	63.996	65.25

Working

M = 2 · 1.0080 + 32.06 + 4 · 15.999 = 98.0720 ≈ 98.07 g/mol

The result has as many decimal places as the least precise atomic mass, and never more than two.

Mass, moles and particles

I know themass m (g)amount n (mol)number of particles NValue

M = 98.07 g/mol. Results are given to 4 significant figures. Avogadro constant: 6.022 140 76 × 10²³ mol⁻¹ (exact).

How do you calculate molar mass?

Molar mass, $M$, is the mass of one mole of a substance, in g/mol (also written $\mathrm{g\,mol^{-1}}$). To work it out, take the relative atomic mass of each element from the periodic table, multiply it by the number of atoms of that element in the formula and add up the results. For sulfuric acid, $\mathrm{H_2SO_4}$, with the IUPAC 2024 values:

In g/mol, molar mass has the same value as the relative formula mass, $M_r$, which has no unit: for sulfuric acid, $M_r = 98.07$. Since the mole was redefined in 2019, the match is no longer exact, but the difference is far too small to matter in practical chemistry.

A number after a bracket multiplies everything inside it: $\mathrm{Ca(OH)_2}$ contains two oxygen atoms and two hydrogen atoms. In a hydrate such as copper(II) sulfate pentahydrate, $\mathrm{CuSO_4 \cdot 5H_2O}$, the water of crystallisation counts too, so its molar mass, 249.68 g/mol, includes the mass of five $\mathrm{H_2O}$ units.

Percentage by mass

The percentage by mass of an element is its share of the molar mass. For sulfur in $\mathrm{H_2SO_4}$:

The calculator divides by the unrounded total (98.072 rather than 98.07) and rounds each percentage to 0.01%, so the percentages can add up to slightly more or less than 100%, e.g. 99.99% or 100.01%.

Mass, moles and particles

The amount of substance, $n$ (in moles), follows from the mass of the sample, $m$, and the number of particles, $N$, from the amount and the Avogadro constant, $N_A$:

The Avogadro constant is exact: since the 2019 revision of the SI it has been fixed at $N_A = 6.022\,140\,76 \times 10^{23}\ \mathrm{mol^{-1}}$. For example, for 9.807 g of sulfuric acid:

The sample contains $N = 0.1000\ \mathrm{mol} \times N_A \approx 6.022 \times 10^{22}$ molecules. The calculator gives these results to 4 significant figures.

Why might the last digit differ?

Periodic tables round atomic masses differently. The calculator uses the IUPAC 2024 values, such as H 1.0080, O 15.999 and S 32.06. Your textbook or exam data sheet may round them more, so check which values it uses and work with those. If your table gives H 1.0, O 16.0 and S 32.1, for example, sulfuric acid comes out at 98.1 g/mol. Some books and databases list it as 98.08 g/mol. The method is the same each time; only the atomic masses differ.

The result has as many decimal places as the least precise atomic mass in the formula, and never more than two. The atomic mass of lead, 207.2, has only one decimal place, so for lead(II) oxide, $\mathrm{PbO}$, the calculator gives 223.2 g/mol.

What won’t the calculator accept?

• Elements with no standard atomic weight, such as technetium (Tc) and plutonium (Pu). They have no stable isotopes and no characteristic isotopic composition on Earth, so IUPAC gives them no standard atomic weight. Periodic tables often show a whole number in square brackets for them instead: the mass number of the longest-lived isotope.
• Formulas typed in lower case. Capitals change the meaning: Co is cobalt, but CO is carbon monoxide. So the calculator rejects “co” and asks you to type the formula correctly rather than guessing.
• A charge typed straight after a number, such as SO42-. It could mean $\mathrm{SO_4^{2-}}$, or 42 oxygen atoms and a single negative charge, so put the charge after a ^: SO4^2-. With no digit before the sign, as in OH-, the ^ is not needed. The calculator ignores the charge anyway: the molar mass of an electron is only about 0.000 55 g/mol, much smaller than the last decimal place of the result (0.01 or 0.1 g/mol).