Moles and Mass Balance

A refresher on the mole and on reaction stoichiometry. Chemists count atoms and molecules in moles: one mole of any substance contains the same number of particles, but its mass depends on the atoms it is made from. First see how the mass of a fixed amount of substance depends on its formula. Then follow a balanced equation from the mass of one substance to the masses of all the others, and see that mass is conserved.

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Amount 1.00 mol

Remember

One mole contains 6.02 × 10²³ particles (atoms, molecules or ions): Avogadro's number.
The molar mass, M, is the sum of the atomic masses in the formula: M(CH₄) = 1 × 12 + 4 × 1 = 16 g mol⁻¹.
Moles = mass ÷ molar mass, n = m ÷ M. Mass = moles × molar mass, m = n × M.
The coefficients in a balanced equation give the ratio of moles that react, not the ratio of masses.
Mass is always conserved in a chemical reaction: the total mass of the products equals the total mass of the reactants.
This lesson uses whole-number atomic masses, as in the lecture slides: H = 1, C = 12, N = 14, O = 16 and Fe = 56 g mol⁻¹. Periodic-table values (for example O = 15.999 g mol⁻¹) give answers that differ slightly.