CaCO₃ Decomposition | IB Chemistry

CaCO₃(s) → CaO(s) + CO₂(g)

CaCO₃ decomposes when heated strongly above ~840°C
CO₂ gas collected over water by downward displacement (inverted measuring cylinder in trough)
Stoichiometry: 1 mol CaCO₃ produces 1 mol CO₂
M(CaCO₃) = 40.08 + 12.01 + 3(16.00) = 100.09 g/mol
At STP (0°C, 100 kPa), 1 mol gas = 22.7 dm³
Collected gas is saturated with water vapor → vapor pressure correction needed

Vapor pressure correction: P_dry = P_atm − P_water — students often forget this step
Combined gas law: P₁V₁/T₁ = P₂V₂/T₂ to convert to STP
% yield = (experimental moles / theoretical moles) × 100
Sig figs: Match measuring instrument precision (±0.5 mL for graduated cylinder)
Complete decomposition: Heat to constant mass — when mass stops changing, all CaCO₃ has decomposed

Gas leaks — delivery tube connections not airtight → lower volume
CO₂ dissolves in water — solubility ~1.5 g/L at 20°C → lower yield
Incomplete decomposition — not heated long enough → less gas
Vapor pressure ignored — overestimates gas volume and yield
Temperature not equilibrated — hot gas expands (Charles's Law) → volume reads high