Steps: (1) Write balanced equation. (2) Convert given quantity to moles. (3) Use mole ratio from balanced equation. (4) Convert to required units. Example: 2H2 + O2 -> 2H2O. Mole ratio: 2 mol H2 : 1 mol O2 : 2 mol H2O. If 4 g H2 reacts: moles H2 = 4/2 = 2 mol. Moles H2O = 2 mol (1:1 ratio). Mass H2O = 2 x 18 = 36 g. For gases: can use volume ratios directly (Gay-Lussac's law). 2 volumes H2 + 1 volume O2 -> 2 volumes H2O (at same T, P). Coefficients in balanced equations represent mole ratios, NOT mass ratios.