What is the difference between molar mass and molecular weight?

They are the same number with different units. Molecular weight (or relative molecular mass) has no units, while molar mass is in grams per mole. Water has a molecular weight of 18.015 and a molar mass of 18.015 g/mol.

Why does capitalization matter in a formula?

Element symbols are case-sensitive. Co is cobalt, but CO is one carbon and one oxygen atom (carbon monoxide). Each symbol starts with a capital letter, and any second letter is lowercase.

Which atomic masses does this calculator use?

The IUPAC standard atomic weights, rounded to the abridged values printed on most periodic tables, such as 1.008 for hydrogen and 15.999 for oxygen. Your textbook may round slightly differently, which can change the last decimal place.

How do I find the molar mass of a hydrate?

Add the molar mass of the water molecules to the compound's. Type it with a dot, like CuSO4·5H2O, and the calculator includes the 5 × 18.015 g/mol of water for you: 249.677 g/mol in total.

Molar Mass Calculator with Steps

Molar mass is the sum of the atomic masses of every atom in the formula. Glucose, C6H12O6, is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol.

Chemical formula

Molar mass of C₆H₁₂O₆

180.156 g/mol

Atoms per unit

Mass by element

Element	Atoms	g/mol	Mass %
C Carbon	6	72.066	40.00%
H Hydrogen	12	12.096	6.71%
O Oxygen	6	95.994	53.28%

Show the math

1
Count the atoms of each element
C₆H₁₂O₆ → C 6, H 12, O 6
Each subscript says how many atoms of the element before it are in one unit.
= 24 atoms

Read the steps as text

Count the atoms of each element. C₆H₁₂O₆ → C 6, H 12, O 6 Each subscript says how many atoms of the element before it are in one unit.
Carbon (C). 6 × 12.011 g/mol = 72.066 g/mol Multiply the number of atoms by the element's atomic mass from the periodic table.
Hydrogen (H). 12 × 1.008 g/mol = 12.096 g/mol
Oxygen (O). 6 × 15.999 g/mol = 95.994 g/mol
Add the contributions together. 72.066 + 12.096 + 95.994 = 180.156

How molar mass is calculated

A chemical formula tells you how many atoms of each element are in one molecule or formula unit. To find the molar mass, multiply each element's count by its atomic mass from the periodic table, then add the results. The answer is the mass of one mole of the substance, in grams per mole (g/mol).

For water, H2O, that's 2 × 1.008 for the two hydrogens plus 15.999 for the oxygen, which comes to 18.015 g/mol. So 18.015 grams of water contains one mole of water molecules, about 6.022 × 10²³ of them.

Brackets, hydrates and percent composition

A number after brackets multiplies everything inside them. In Ca(OH)2 the 2 applies to both the O and the H, giving 1 calcium, 2 oxygen and 2 hydrogen atoms. Square brackets work the same way, so K4[Fe(CN)6] is read correctly too.

Hydrates are written with a dot: CuSO4·5H2O means one copper sulfate unit plus five water molecules. You can type the dot as ·, a full stop or an asterisk. The number after the dot multiplies the whole water molecule, so it adds 10 hydrogen and 5 oxygen atoms.

The table under the result shows each element's share of the total mass, its percent composition. Each element's contribution is divided by the molar mass and multiplied by 100, so in water hydrogen is 2.016 ÷ 18.015 × 100 = 11.19% by mass and oxygen is the other 88.81%.

Frequently asked questions

What is the difference between molar mass and molecular weight?: They are the same number with different units. Molecular weight (or relative molecular mass) has no units, while molar mass is in grams per mole. Water has a molecular weight of 18.015 and a molar mass of 18.015 g/mol.
Why does capitalization matter in a formula?: Element symbols are case-sensitive. Co is cobalt, but CO is one carbon and one oxygen atom (carbon monoxide). Each symbol starts with a capital letter, and any second letter is lowercase.
Which atomic masses does this calculator use?: The IUPAC standard atomic weights, rounded to the abridged values printed on most periodic tables, such as 1.008 for hydrogen and 15.999 for oxygen. Your textbook may round slightly differently, which can change the last decimal place.
How do I find the molar mass of a hydrate?: Add the molar mass of the water molecules to the compound's. Type it with a dot, like CuSO4·5H2O, and the calculator includes the 5 × 18.015 g/mol of water for you: 249.677 g/mol in total.