n = mass / M
The mole is chemistry's counting unit: 1 mol = 6.022 × 10²³ particles (Avogadro's number). To convert grams to moles: n = mass/M, where M is the molar mass in g/mol (numerically equal to the molecular or formula weight in amu). To convert moles to grams: mass = n × M. This conversion is the foundation of stoichiometry — it connects measurable masses to the number of atoms/molecules reacting. For example, 100 g of water = 100/18.015 = 5.55 mol = 3.34 × 10²⁴ molecules. The molar mass is calculated by summing atomic masses from the periodic table. For ionic compounds, use the formula mass. Accurate molar mass values are critical for preparing solutions of known concentration and for yield calculations.