Analytical Chemistry
Analytical chemistry in JAM CY is about reliable measurement: error and statistics, acid–base and redox titrations, spectrophotometry, chromatography, and electroanalytical methods. The exam tests whether you can choose a method, calculate a result with correct units, and distinguish precision from accuracy.
Core rules
- Errors: systematic errors shift results in one direction; random errors scatter. Precision describes reproducibility; accuracy describes closeness to the true value. Report standard deviation for replicate measurements.
- Titrations: at equivalence, moles of titrant equal moles of analyte according to stoichiometry. For a monoprotic acid–base titration, M_a V_a = M_b V_b only when the reaction is 1:1.
- Beer–Lambert law: A = εbc, where A is absorbance (dimensionless), ε is molar absorptivity (L mol−1 cm−1), b is path length (cm), and c is concentration (mol L−1). Absorbance is additive for non-interacting species.
- Chromatography: retention time identifies, peak area quantifies; resolution depends on selectivity, efficiency, and retention factor. Plate number N = 16(t_R/w)².
- Electrochemistry: Nernst equation E = E° − (0.0592/n) log Q at 25 °C for base-10 log.
Worked example
A solution has absorbance 0.450 in a 1.00 cm cell; ε = 1.50 × 10^4 L mol−1 cm−1. Find c. c = A/(εb) = 0.450/[(1.50 × 10^4)(1.00)] = 3.00 × 10^−5 mol L−1. The reasoning is to rearrange Beer–Lambert and keep b in cm and ε in L mol−1 cm−1 so the units cancel to mol L−1.
Traps
Using path length in mm while ε is per cm; forgetting the blank/background absorbance; confusing endpoint with equivalence point; carrying too many significant figures; and treating a systematic error as fixable by more replicates.