Chemistry happens between particles — atoms and molecules — but we can only weigh things in grams. The mole bridges that gap: it is a fixed, huge count of particles (6.022×10²³) chosen so that one mole of any substance weighs its molar mass in grams. So to turn a weighed mass into a usable particle count, you divide by the molar mass: n = m/M. That one step is the foundation of stoichiometry, letting you scale recipes, predict yields and make solutions of exactly the right strength.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the mole concept and Avogadro's number.
The mole equations
Mass m is in grams and molar mass M in grams per mole, so the moles n are dimensionless counts of "how many molar units." The molar mass is read straight from the formula — add the atomic masses of every atom — which is why water (18 g/mol) and carbon dioxide (44 g/mol) have the values they do. Multiplying moles by Avogadro's number scales up to the actual number of atoms or molecules, a figure far too large to count any other way.
Worked example — weighing out salt
Scenario: You weigh 58.44 g of sodium chloride (NaCl), molar mass 58.44 g/mol. How many moles and formula units is that?
Exactly one mole — 6.022×10²³ NaCl units. Weigh out half that, 29.22 g, and you have 0.5 mol; weigh 117 g and you have ~2 mol, because moles scale directly with mass. If instead you knew you needed 0.25 mol of water, the mass would be n×M = 0.25 × 18.0 = 4.5 g. The molar mass is the single conversion factor running both directions.
Frequently Asked Questions
n = m / M (mass ÷ molar mass). 58.44 g NaCl ÷ 58.44 g/mol = 1 mol.
A count of 6.022×10²³ particles (Avogadro's number), defined so one mole weighs the molar mass in grams.
moles = grams ÷ molar mass; reverse: grams = moles × molar mass. 18 g water ÷ 18 g/mol = 1 mol.
Mass of one mole (g/mol), = sum of atomic masses in the formula. H₂O = 18.0, CO₂ = 44.0 g/mol.
6.022×10²³. Convert mass → moles (n=m/M), then × Avogadro's number for the particle count.