Molar Mass Calculator

What it is and when to use it

Molar mass is the mass in grams of one mole, or about 6.022 x 10²³ units, of a substance. It is the bridge between the counting world of chemical equations and the weighing world of the laboratory balance. Every stoichiometry problem that starts with a mass and ends with moles, or the reverse, needs it.

Enter a formula and the calculator adds up the standard atomic weight of every atom, handles parentheses and brackets, and supports hydrate notation. It also shows how much each element contributes, which makes it useful for percent composition problems, for preparing solutions of a required molarity, and for checking that a formula you typed from a label is the compound you intended.

The formula and how it works

The molar mass M of a compound is the sum over its elements:

M = sum of (n_i x A_i), where n_i is how many atoms of element i are in the formula and A_i is its atomic weight in g/mol.
A number after a parenthesis multiplies everything inside, so Ca(OH)2 contains one Ca, two O and two H.
A hydrate coefficient multiplies the whole water group, so 5H2O adds ten H and five O.
Mass percent of element i = (n_i x A_i) / M x 100.

Worked example

Take calcium hydroxide, Ca(OH)2. The formula holds one calcium, two oxygen and two hydrogen atoms. Using Ca = 40.078, O = 15.999 and H = 1.008: calcium contributes 40.078, oxygen contributes 2 x 15.999 = 31.998, and hydrogen contributes 2 x 1.008 = 2.016.

Adding them gives 40.078 + 31.998 + 2.016 = 74.092 g/mol. The mass percentages are 40.078 / 74.092 = 54.09 percent calcium, 43.19 percent oxygen and 2.72 percent hydrogen, which sum to 100. As a second check, water H2O gives 2 x 1.008 + 15.999 = 18.015 g/mol, the calculator output for that formula.

Common mistakes and how to interpret the result

Getting capitalization wrong: Co and CO, or Si and SI, are different things, and the parser treats them that way.
Forgetting that a subscript after a closing bracket applies to every atom inside it, which is the usual cause of wrong answers with formulas like Al2(SO4)3.
Dropping the water of crystallization: a hydrate weighs much more than the anhydrous salt, so use the right form when weighing out a reagent.
Over-reporting precision: atomic weights carry only four or five significant figures, so quote results consistent with your data rather than to many extra decimals.

Frequently Asked Questions

How do I type hydrates such as copper sulfate pentahydrate?

Write the anhydrous part, a separator, then the water with its coefficient, for example CuSO4.5H2O. The calculator accepts a period, a middle dot or an asterisk as the separator and multiplies everything after it by the leading number. CuSO4.5H2O comes out to about 249.677 g/mol.

Are element symbols case-sensitive?

Yes. A capital letter starts every element symbol and any following lowercase letter belongs to it. Co is cobalt while CO is one carbon and one oxygen, and Nacl is not a valid symbol. If you see an unknown-element message, check your capitalization first.

Which atomic weights does it use?

It uses conventional standard atomic weights rounded to three or four significant decimals, such as 1.008 for hydrogen, 12.011 for carbon and 15.999 for oxygen. Elements with variable natural composition, like chlorine or boron, use the common abbreviated value. Radioactive elements without a stable standard weight are not included.

What is the difference between molar mass and molecular weight?

Molar mass is the mass of one mole of a substance in grams per mole. Molecular weight, or relative molecular mass, is the same number without units, relative to one twelfth of a carbon-12 atom. For ionic compounds like NaCl the term formula mass is more accurate than molecular weight, but the numerical value is identical.