What's the difference between molar mass and molecular weight?

In practice they're used interchangeably. Molar mass is the mass of one mole of a substance in g/mol; molecular weight (MW) is the dimensionless sum of atomic weights. For any real-world calculation the numeric values are identical, so this tool labels the output g/mol.

How do I write a hydrate?

Use ·, *, or . as the separator between the anhydrous formula and the water group, and prefix the water with the number of waters. Examples: CuSO4·5H2O, MgSO4*7H2O, Na2CO3.10H2O. Every one parses to the same result.

Why is the case sensitive?

Chemistry symbols are Capitalized: 'Co' is cobalt but 'CO' is carbon monoxide (C + O). If you type 'co' the parser can't tell what you meant, so it errors instead of guessing wrong. Same reason organic shorthand like 'Me' (methyl), 'Et' (ethyl), 'Ph' (phenyl) isn't supported — expand to the full formula.

Does the calculator handle charges or isotopes?

No. Charge is dropped in formula-mass calculations (an ion has the same molar mass as its neutral form, within a fraction of an electron mass we can ignore). Isotope notation like ^13C is rejected; the calculator uses the IUPAC conventional atomic weight for the natural-abundance mixture.

Molar Mass Calculator — Formula to g/mol

Every stoichiometry, dilution, and buffer calculation starts with a molar mass. Type the formula in Hill notation and the calculator returns MW in g/mol plus a per-element breakdown so you can double-check the arithmetic.

Molar Mass Calculator

Type a chemical formula in Hill notation. Case-sensitive: "Na" not "na" or "NA". Parentheses, brackets, hydrates (·, *, .) and a leading integer coefficient are all supported.

How it works

For a compound with formula X_aY_b…, the molar mass is the sum of atomic weights weighted by counts:

$MW = \sum_i n_i \, A_i$

where n_i is the number of atoms of element i in the formula unit and A_i is that element’s atomic weight (g/mol). The calculator uses the IUPAC 2021 conventional atomic weights — the single-value form recommended for teaching and general lab use.

Parentheses and brackets multiply through: Ca(OH)2 = one Ca + 2×(O+H) = one Ca + 2 O + 2 H. Hydrate separators (·, *, .) join anhydrous salt to its bound waters: CuSO4·5H2O = one CuSO4 + 5×H2O. A leading integer treats the whole formula as a coefficient: 3H2O = 6 H + 3 O.

Common lab reagents

Formula	MW (g/mol)	Notes
NaCl	58.44	Sodium chloride, physiological saline base
KCl	74.55	Potassium chloride
NaOH	40.00	Sodium hydroxide, hygroscopic
HCl (aq, concentrated)	36.46	~12 M in solution
H2SO4	98.08	Concentrated is ~18 M
Na2CO3	105.99	Anhydrous sodium carbonate
NaHCO3	84.01	Sodium bicarbonate
Tris (base)	121.14	Buffer, pKa 8.06
HEPES	238.30	Buffer, pKa 7.55
D-Glucose	180.16	Anhydrous
Sucrose	342.30
EDTA (free acid)	292.24	Chelator; Na2EDTA·2H2O is 372.24
CuSO4·5H2O	249.68	Blue vitriol; anhydrous is 159.61
MgSO4·7H2O	246.47	Epsom salt

Values from PubChem and IUPAC 2021 atomic weights. If a bottle label disagrees, trust the label — atomic-weight uncertainty is real at 4+ sig figs.

Common pitfalls

Case matters. Co is cobalt, CO is carbon monoxide. co and CO2 are two very different things. The parser errors on lowercase-first symbols instead of guessing.
Hydrates aren’t optional. A “CuSO4” bottle labeled “pentahydrate” is 249.68 g/mol, not 159.61. If you use the anhydrous MW you’ll weigh 36 % too little.
Organic shorthand isn’t supported. Write CH3OH not MeOH; C6H5CH3 not PhCH3. Expanded formulas are unambiguous.
Charges are ignored. The formula mass of Na+ is the same as neutral Na (electron mass is ~0.0005 g/mol — well below our precision). Don’t include the charge in the input.

Practice problems

Attempt each on paper, then expand the worked solution to check your arithmetic and sig-figs.

Problem 1 Easy What is the molar mass of H2SO4?

Answer: 98.079 g/mol

Sum contributions element by element

H: 2 × 1.008 = 2.016
S: 1 × 32.06 = 32.06
O: 4 × 15.999 = 63.996

Formula
$M W = \sum n_{i} A_{i}$
Substitute
$M W = 2.016 + 32.06 + 63.996$
Note
Small discrepancy vs the PubChem 98.079 comes from IUPAC-2021 conventional atomic weights being slightly less precise than the isotopic-abundance-weighted mean; both are correct within measurement uncertainty.
Result
$M W = 98.072 g/mol$

Problem 2 Intermediate What is the molar mass of Al2(SO4)3?

Answer: 342.15 g/mol

Distribute the ×3 across the parenthesised group

Al: 2 × 26.982 = 53.964
S: 3 × 32.06 = 96.18
O: 12 × 15.999 = 191.988

Formula
$M W = 2 A_{A l} + 3 A_{S} + 3 \cdot 4 A_{O}$
Substitute
$M W = 53.964 + 96.18 + 191.988$
Result
$M W = 342.13 g/mol$

Problem 3 Intermediate What is the molar mass of CuSO4·5H2O (the pentahydrate)?

Answer: 249.68 g/mol

Include the five water molecules in the mass

CuSO4 (anhydrous): 63.546 + 32.06 + 4·15.999 = 159.61
5 H2O: 5 × 18.015 = 90.075

Formula
$M W_{hydrate} = M W_{anhy} + n \cdot M W_{H_{2} O}$
Substitute
$M W = 159.61 + 90.075$
Note
The five waters count toward the mass you weigh out. Using the anhydrous value (159.61) would under-weigh by 36 %.
Result
$M W = 249.68 g/mol$

Problem 4 Hard What is the molar mass of K3[Fe(CN)6] (potassium ferricyanide)?

Answer: 329.24 g/mol

Nested brackets: 6 cyanides on the iron, ×3 potassium

K: 3 × 39.098 = 117.294
Fe: 1 × 55.845 = 55.845
C: 6 × 12.011 = 72.066
N: 6 × 14.007 = 84.042

Formula
$M W = 3 A_{K} + A_{F e} + 6 A_{C} + 6 A_{N}$
Substitute
$M W = 117.294 + 55.845 + 72.066 + 84.042$
Result
$M W = 329.25 g/mol$

Frequently asked questions

What's the difference between molar mass and molecular weight?: In practice they're used interchangeably. Molar mass is the mass of one mole of a substance in g/mol; molecular weight (MW) is the dimensionless sum of atomic weights. For any real-world calculation the numeric values are identical, so this tool labels the output g/mol.
How do I write a hydrate?: Use ·, *, or . as the separator between the anhydrous formula and the water group, and prefix the water with the number of waters. Examples: CuSO4·5H2O, MgSO4*7H2O, Na2CO3.10H2O. Every one parses to the same result.
Why is the case sensitive?: Chemistry symbols are Capitalized: 'Co' is cobalt but 'CO' is carbon monoxide (C + O). If you type 'co' the parser can't tell what you meant, so it errors instead of guessing wrong. Same reason organic shorthand like 'Me' (methyl), 'Et' (ethyl), 'Ph' (phenyl) isn't supported — expand to the full formula.
Does the calculator handle charges or isotopes?: No. Charge is dropped in formula-mass calculations (an ion has the same molar mass as its neutral form, within a fraction of an electron mass we can ignore). Isotope notation like ^13C is rejected; the calculator uses the IUPAC conventional atomic weight for the natural-abundance mixture.

Sources

Last updated: 2026-08-13.