Classical Wet Chemical and Gravimetric/Volumetric Methods
Classical Wet Chemical and Gravimetric/Volumetric Methods
This domain tests stoichiometric calculations and the physical technique behind them: primary standards, standardization, indicators, back titrations, precipitation, and weighing to constant mass.
Core rules
- A primary standard is high purity, stable, non-hygroscopic and often has a high molar mass. KHP standardizes NaOH; sodium carbonate standardizes acids; potassium dichromate is used in redox work.
- The equivalence point is stoichiometric; the endpoint is observed. Choose an indicator whose transition brackets the equivalence pH or potential.
- n(analyte) follows the balanced equation, not a guessed 1:1 ratio. Back titration: analyte = total titrant − excess titrant.
- Gravimetric precipitates must be filterable, pure, and of known composition after ignition. Co-precipitation and post-precipitation cause positive bias; peptization causes loss.
- Weigh to constant mass, with successive weights agreeing within balance tolerance. Burette readings are estimated to about ±0.05 mL; use the difference, not the final reading.
Worked example
0.5120 g KHP (204.22 g/mol) is titrated to a phenolphthalein endpoint with 24.85 mL NaOH. Moles KHP = 0.5120/204.22 = 2.507 × 10^-3 mol. At 1:1 stoichiometry, M(NaOH) = 2.507 × 10^-3 / 0.02485 L = 0.1009 mol/L. If the initial burette reading was 0.05 mL, the delivered volume is 24.80 mL; use that difference.
Traps
- Using the final burette reading instead of the volume delivered.
- Drying KHP insufficiently or weighing hygroscopic NaOH.
- Forgetting the stoichiometric ratio in redox or complexometric titrations.
- Ignoring the indicator blank.
- Weighing a hygroscopic precipitate without a desiccator.