Exams › JEE Advanced › Chemistry › Purification and Characterisation of Organic Compounds
4 questions with worked solutions.
Q1. The most suitable method for separating a mixture of naphthalene and benzoic acid is
Answer: Sublimation
Naphthalene is a volatile solid that sublimes readily (sublimation point ~80 deg C). Benzoic acid also sublimes but requires higher temperature. In practice, gentle heating of the mixture causes naphthalene to sublime first and collect on a cold surface, separating it from benzoic acid. Sublimation is the standard method taught for separating such mixtures in NCERT chemistry.
Q2. Which of the following statements is correct regarding the stationary phase in paper chromatography?
Answer: Water present in the pores of the paper forms the stationary phase.
Paper chromatography is a type of partition chromatography. The paper (cellulose) contains a thin film of water adsorbed in its pores, which acts as the stationary phase. The mobile phase is an organic solvent (or a mixture) that travels up the paper. Compounds distribute between the mobile solvent and the stationary water layer, separating based on their partition coefficients. It is the water in the pores, not the paper itself, that constitutes the stationary phase.
Answer: Fractional distillation
Fractional distillation is the standard method when two miscible liquids differ in boiling point by less than 20 degrees C; the fractionating column gives repeated equilibration stages. Steam distillation is for immiscible high-boiling compounds, and vacuum distillation is for heat-sensitive ones.
Answer: Steam distillation
Aniline (bp 184 deg C) is immiscible with water and would decompose near its boiling point. Steam distillation lets it co-distil with steam at about 98 deg C, well below its normal boiling point, without decomposition.