Solution Stoichiometry & Titrations
n = MV stoichiometry, acid-base and redox titrations, standards, purity, and gravimetric analysis.
Balance first
CORE RULEA titration is stoichiometry in solution. Write and balance the reaction before any calculation.
n = MV
CORE RULEMoles from a solution: n = M × V (V in litres). Rearrange: V = n / M. Use it wherever a mass-based path would use molar mass.
Titration vocabulary
REFERENCETitrant: known solution from the buret. Analyte: unknown being measured. Equivalence point: titrant volume delivering the exact stoichiometric amount (equal moles only for a 1:1 reaction). Endpoint: measured volume (indicator colour change). Primary standard: high-purity weighable solid (e.g. KHP).
KHP acid standard
CORE RULEKHP (KHC₈H₄O₄, 204.22 g/mol) is a monoprotic solid acid that reacts 1:1 with NaOH; used to standardize base solutions.
Polyprotic neutralization
CORE RULEPolyprotic acids neutralize stepwise (a separate equivalence point per proton). At complete neutralization: H₃PO₄ needs 3 NaOH; H₂SO₄ and H₂C₂O₄ each need 2 NaOH.
Unknown concentration
METHODmol titrant (M × V) → mol analyte (stoichiometric factor) → divide by analyte volume: M = n / V.
Purity of a solid
METHODTitration → mass of pure analyte; percent purity = (mass analyte / total sample mass) × 100.
Redox titrations
REFERENCE| Titrant | Balanced half-context (acidic) |
|---|---|
| K₂Cr₂O₇ | Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺ |
| KMnO₄ | MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O |
Limiting reactant in solution
CORE RULESame as with pure substances, but moles come from n = MV. Compare each reactant's moles against the balanced ratio to find the limiting one.
Precipitation & gravimetric
METHODIdentify the precipitate (solubility rules), find moles of each reactant, pick the limiting one, then compute precipitate mass.
Gravimetric: mass of precipitate → mol precipitate (÷ MM) → mol analyte (ratio) → mass analyte; mass % = (mass analyte / sample mass) × 100.
Hydrated reagents
WATCH OUTFor a hydrated base (e.g. LiOH·H₂O), molar mass includes the water, but the stoichiometry uses the base formula, so one mole of hydrate gives one mole of LiOH.