By Kristen Gonsoir | Updated August 30, 2022
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In chemistry, determining the exact count of representative particles—whether atoms, molecules, formula units, or ions—is essential for accurate stoichiometric calculations. Every substance has a well‑defined chemical composition reflected in its formula. The standard unit for quantifying a substance’s amount is the mole; one mole contains 6.02 × 10²³ particles—known as Avogadro’s number.
Weigh a sample of the substance and record its mass in grams. For instance, a 36.0‑gram sample of water.
Compute the molar mass by summing the average atomic masses of the constituent atoms from the periodic table. Water’s molar mass is 18.0 g mol⁻¹ (2 × 1.0 g for H + 16.0 g for O).
Divide the measured mass by the molar mass: 36.0 g ÷ 18.0 g mol⁻¹ = 2 mol of water.
Multiply the number of moles by Avogadro’s number: 2 mol × 6.02 × 10²³ particles mol⁻¹ = 1.20 × 10²⁴ particles.
The precision of the final particle count is limited by the least precise measurement—in this case, the mass. If you already know the moles, simply multiply by 6.02 × 10²³ to obtain the particle count.
