Molar Mass Calculator

What is molar mass?

Molar mass is the mass of one mole of a substance, measured in grams per mole. You find it by adding up the atomic masses of every atom in the chemical formula. It is the bridge between the grams you weigh out on a balance and the moles that chemical equations actually deal in.

In words, multiply each element's atomic mass by how many atoms of it appear, then add the results together.

How to use this calculator

1. Enter the chemical formula, such as \(\\text{H}_2\\text{O}\) or \(\\text{CaCl}_2\).
2. Use the periodic table values for each element’s atomic mass, then multiply by how many atoms of that element appear in the formula.
3. Add the contributions from all elements to get the total molar mass.
4. Check the result in \(\\text{g/mol}\), then use it for mole to mass calculations.

The formula explained

The formula \(M = \\sum n_i \\times M_i\) computes the total molar mass by adding the mass contribution of each element in the compound. Here, each element’s atomic mass is multiplied by how many atoms of that element are present.

• M = the total molar mass of the compound in \(\\text{g/mol}\)
• \(n_i\) = the number of atoms of element \(i\) in the chemical formula
• \(M_i\) = the atomic mass of element \(i\), usually in \(\\text{g/mol}\)
• i = an index that labels each different element in the compound

Step by step method

1. Write the compound’s formula and identify each element, along with how many atoms of each element appear.
2. Look up the atomic mass of each element, then multiply each atomic mass by its atom count.
3. Add all of the element contributions together to get the molar mass.

Worked example

Problem. Find the molar mass of \(\\text{H}_2\\text{O}\).

1. Hydrogen has atomic mass \(1.008\\,\\text{g/mol}\), and oxygen has atomic mass \(15.999\\,\\text{g/mol}\).
2. Compute the contributions, \(2 \\times 1.008 = 2.016\\,\\text{g/mol}\) for hydrogen and \(1 \\times 15.999 = 15.999\\,\\text{g/mol}\) for oxygen.
3. Add them: \(2.016 + 15.999 = 18.015\\,\\text{g/mol}\), so the molar mass of water is \(18.015\\,\\text{g/mol}\).

Tips and common mistakes

• Be careful to count every atom in the formula, especially when there are parentheses or subscripts.
• Do not round too early, because small rounding errors can change the final molar mass.

Frequently asked questions