Chemical Calculations, Stoichiometry and Quantitative Relationships
What you must understand
Master the mole concept: n = m/M, n = N/N_A, and for gases at standard conditions n = V/V_m. Know molar volume and Avogadro's constant. Use concentration c = n/V and dilution c₁V₁ = c₂V₂. Balance equations and read coefficients as mole ratios. Identify limiting reagent, theoretical yield and percentage yield, plus purity.
Empirical and molecular formulae: convert mass percentages to moles, divide by the smallest, then scale by molar mass. Know percentage composition by mass. For solutions, distinguish solute, solvent, mass fraction w, and molar concentration.
Traps
Unbalanced equations ruin every ratio. Units matter - convert cm³ to dm³ (÷1000). Molar volume applies only to gases and only at the stated conditions. Don't confuse mass fraction (unitless or %) with concentration (mol/dm³). When a limiting reagent is present, always test which reactant runs out first.
Rule of thumb
Write the balanced equation, convert everything to moles, apply the mole ratio, then convert back to the requested quantity. Track units at every step.
Worked example
How many grams of water form when 4 g of H₂ reacts with excess O₂? M(H₂) = 2 g/mol, so n(H₂) = 2 mol. 2H₂ + O₂ → 2H₂O gives n(H₂O) = 2 mol. M(H₂O) = 18 g/mol, so m = 36 g.