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Purification & Characterisation: one-page cheat sheet

NEET Chemistry · Unit 13 · Organic Chemistry

ScoreChemby CSJ Sir
Weightage: 1–2 questions · 4–8 marks

1 · Purification methods

Method	Use
Sublimation	camphor, naphthalene, benzoic acid
Crystallisation	sugar, benzoic acid (hot water)
Simple distillation	$Δ$ b.p. $> 25$ K: CHCl $_{3}$ + aniline
Fractional distillation	$Δ$ b.p. $< 25$ K: crude oil
Reduced pressure	glycerol from spent-lye
Steam distillation	aniline–water; essential oils
Differential extraction	organic solute from water

Steam distillation: boils when $p_{1} + p_{2} = p_{atm}$ , so below 373 K.

2 · Chromatography

Adsorptioncolumn and TLC (silica gel, alumina)

Partitionpaper chromatography (water in cellulose)

R_{f} = \frac{distance moved by substance}{distance moved by solvent front} \leq 1

SpotsUV light; iodine vapour; ninhydrin for amino acids

3 · Lassaigne's test

Fuse with Na: covalent N, S, X become ionic NaCN, Na $_{2}$ S, NaX.

Element	Test	Result
N	FeSO $_{4}$ , then acid + Fe $^{3 +}$	Prussian blue
S	lead acetate / nitroprusside	black / violet
N + S	FeCl $_{3}$ (NaSCN)	blood red
X	HNO $_{3}$ , then AgNO $_{3}$	AgCl white; AgBr pale yellow; AgI yellow
P	Na $_{2}$ O $_{2}$ , HNO $_{3}$ , molybdate	canary yellow

4 · Exceptions

C, Hheat with CuO: CO

_{2}

(lime water) and H

_{2}

O. Not part of Lassaigne's test

No Chydrazine gives no NaCN, no Prussian blue

Boil with HNO

_{3}

first, if N or S is present, to avoid AgCN and Ag

_{2}

Excess NaNaSCN splits: no blood red

Phthalein testidentifies the phenolic group

5 · Estimation of C, H, N

% C = \frac{12 m _{CO_{2}} \times 100}{44 m}, % H = \frac{2 m _{H_{2} O} \times 100}{18 m}

DumasN

_{2}

collected over conc. KOH at STP

% N = \frac{28 V _{STP} \times 100}{22400 m}

KjeldahlN

\to

(NH

_{4}

)

_{2}

_{4}

\to

_{3}

; back titration

% N = \frac{1.4 \times M \times 2 ( V - V _{1} /2 )}{m}

Kjeldahl fails for nitro, azo and ring nitrogen (pyridine).

6 · Estimation of X, S, P

% X = \frac{at. mass of X \times m _{AgX} \times 100}{molar mass of AgX \times m}

% S = \frac{32 m _{BaSO_{4}} \times 100}{233 m}, % P = \frac{62 m _{Mg_{2} P_{2} O_{7}} \times 100}{222 m}

Cariusfuming HNO

_{3}

in a sealed tube; AgX or BaSO

_{4}

Oxygenby difference:

100 - \sum (others)

AgCl 143.5, AgBr 188, AgI 235.

7 · Empirical formula

Step 1% as grams in 100 g; divide by atomic mass

Step 2divide all by the smallest; clear fractions (×2, ×3)

n = \frac{molar mass}{empirical formula mass}, MF = (EF)_{n}

Vapour density: molar mass $= 2 \times V.D.$

Seen in NEET 2024

32% A, 20% B, rest C (A = 64, B = 40, C = 32).

\frac{32}{64} : \frac{20}{40} : \frac{48}{32} = 1 : 1 : 3 \Rightarrow ABC_{3}

Seen in NEET 2025

Mixture	Method
CHCl $_{3}$ + aniline	simple distillation
crude oil	fractional distillation
glycerol from spent-lye	reduced pressure
aniline–water	steam distillation

Also: $2 CuO + C \to 2 Cu + CO_{2}$ is the carbon test, not Lassaigne's.

Seen in NEET 2020, 2024

Paper chromatography is partition chromatography (water in cellulose is the stationary phase).

Solids that vaporise without melting are purified by sublimation.

Seen in NEET 2026

Lassaigne's test converts elements from covalent to ionic form.

The phthalein dye test identifies the phenolic group.

Seen in NEET 2022

Kjeldahl works for aniline (amino N) but not nitrobenzene, azobenzene or pyridine.

Also 2023: with N and S together, NaSCN gives blood-red $[Fe(SCN)]^{2 +}$ .

Six traps to remember

Paper chromatography is partition chromatography.
Lassaigne's test turns covalent N, S, X into ionic salts.
N and S together give blood red $[Fe(SCN)]^{2 +}$ , not Prussian blue.
Kjeldahl's method fails for nitro, azo and pyridine-type nitrogen.
The phthalein dye test identifies phenols.
$R_{f}$ is always $\leq 1$ because the solvent front moves farther.

Purification and Characterisation of Organic Compounds: free one-page cheat sheet