JEE Chemistry Volumetric Analysis Notes Set 01

Concentration Terms

1. Normality = equivalents of solute per litre of solution. Milliequivalents = N x V (in mL).
2. Molarity = moles per litre; for an acid or base, normality = molarity x n-factor (basicity or acidity).
3. At the end point of a neutralisation: milliequivalents of acid = milliequivalents of base, so \( N_1V_1 = N_2V_2 \).
4. Dilution: \( N_1V_1 = N_2V_2 \) (or \( M_1V_1 = M_2V_2 \)). For mixing solutions of the same kind of species, the final normality = total milliequivalents / total volume.

Equivalent Weights

Indicators and Titrations

1. Methyl orange changes colour at about pH 3 to 4.5 and is suitable for strong acid against weak base titrations. Phenolphthalein changes at about pH 8 to 10 and suits weak acid against strong base.
2. Strong acid against strong base: either indicator is suitable. Weak acid against weak base: no sharp end point.
3. Permanganate titrations are self-indicating (the first permanent pink colour marks the end point) and are done in dilute sulphuric acid. Never use HCl, which is oxidised, or nitric acid, which is itself an oxidant.
4. Iodometric titration: an oxidant (such as dichromate or copper(II)) liberates iodine from excess iodide in acid, and the iodine is titrated with thiosulphate using starch near the end point: \( \ce{2S2O3^{2-} + I2 -> S4O6^{2-} + 2I^-} \).
5. Standard solutions: oxalic acid, potassium dichromate and sodium carbonate are primary standards. NaOH and KMnO4 are not, because NaOH absorbs \( \ce{CO2} \) and water from air and permanganate decomposes slowly.