Mass to Mole Calculator: Grams to Moles From the Molar Mass

This calculator turns a mass in grams into an amount of substance in moles. Enter the mass you weighed and the molar mass of the compound; you get the moles, the same amount in millimoles and the number of formula units (molecules, or ion pairs for a salt).

Formula

n = m ÷ M, and N = n × N_A.

• n: amount of substance in moles (mol)
• m: mass in grams (g)
• M: molar mass in grams per mole (g/mol), the sum of the atomic weights in the formula
• N: number of formula units; N_A = 6.02214076 × 10²³ mol⁻¹ (the Avogadro constant, exact)
• 1 mol = 1000 mmol

Worked Example

25 g of sodium chloride, NaCl, with a molar mass of 58.44 g/mol (22.990 for Na + 35.45 for Cl).

1. n = 25 ÷ 58.44 = 0.4278 mol
2. In millimoles: 0.42779 × 1000 = 427.79 mmol
3. Formula units: 0.42779 × 6.02214076 × 10²³ = 2.576 × 10²³

Assumptions

• The molar mass is that of the exact substance you weighed, including any water of crystallisation (CuSO₄·5H₂O, not CuSO₄).
• The sample is pure; for a reagent of known purity, multiply the mass by the purity fraction first.
• Masses are in grams; convert milligrams (÷ 1000) or kilograms (× 1000) before you enter them.

How to read the result

The amount in moles is what reaction equations and concentrations are written in, so it is the number to carry into a stoichiometry or molarity calculation. The number of formula units is mainly a check of scale: even a few milligrams hold around 10¹⁹ to 10²⁰ of them. Use the molar mass calculator if you do not have M yet.

Limits and common mistakes

• Using the anhydrous molar mass for a hydrate (or the reverse) is the most common error and can be off by 30% or more.
• Entering milligrams as grams gives an answer 1000 times too large.
• For a solution, the mass of solute is not the mass of the solution; use a molarity calculator instead.

Frequently Asked Questions

How many moles are in 25 g of NaCl?

About 0.4278 mol, using M = 58.44 g/mol.

What is the formula for grams to moles?

Moles = grams ÷ molar mass (g/mol).

Where do I find the molar mass?

Add the atomic weights of every atom in the formula, or type the formula into the molar mass calculator on the home page.

Does the result count molecules or atoms?

Formula units: molecules for a molecular compound, one cation-anion set for a salt such as NaCl. For atoms, multiply by the number of atoms in the formula.

in m | Mass of the sample | g | 25 | 0
in mm | Molar mass | g/mol | 58.44 | 0.001
out n | Amount of substance | mol | m / mm | 4
out mmol | Amount in millimoles | mmol | n * 1000 | 2
out nn | Number of formula units | - | n * 6.02214076e23 | 3