Acids, Bases, Salts and Volumetric Analysis
Acids, bases, salts and volumetric analysis
This domain tests acid-base definitions, pH, indicators, salt preparation and titration calculations. GCE O-Level practical questions often ask why a particular indicator or salt-preparation method is chosen.
Core rules
- An acid is a proton donor; a base is a proton acceptor. A strong acid ionises almost completely in water; a weak acid ionises only partially.
- pH below 7 is acidic, 7 is neutral and above 7 is alkaline. Universal indicator gives approximate pH; litmus shows acid or alkali only; phenolphthalein is colourless in acid and pink in alkali; methyl orange is red in acid and yellow in alkali.
- Neutralisation: acid + base -> salt + water. Acid + metal -> salt + hydrogen. Acid + carbonate -> salt + water + carbon dioxide.
- Soluble salts can be made by titration, by reacting an acid with excess insoluble base or carbonate, or by reacting an acid with a reactive metal. Insoluble salts are made by precipitation.
- In titration, moles = concentration x volume in dm3, then use the equation ratio.
Worked example
25.0 cm3 of sodium hydroxide solution is neutralised by 20.0 cm3 of 0.100 mol/dm3 hydrochloric acid. Calculate the concentration of the sodium hydroxide.
Reasoning: HCl + NaOH -> NaCl + H2O, ratio 1:1. Moles HCl = 0.100 x 20.0/1000 = 0.00200 mol. Moles NaOH = 0.00200 mol. Concentration NaOH = 0.00200/(25.0/1000) = 0.0800 mol/dm3. The titre must be converted to dm3 before multiplying by concentration.
Traps that cost marks
Do not use cm3 directly in concentration calculations. Choose an indicator whose colour change matches the titration; phenolphthalein is suitable for strong acid-strong alkali but not for weak acid-strong alkali in all cases. Do not use the wrong acid for the salt: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates. Excess insoluble base is filtered off; excess soluble base would contaminate the salt.
Key points
- Acids are proton donors; bases are proton acceptors.
- Strong acids ionise completely; weak acids do not.
- Neutralisation produces salt and water.
- Titration uses moles = concentration x volume in dm3.
- Salt preparation depends on solubility.