Steam Distillation

Steam distillation purifies substances that are insoluble in water but are appreciably volatile in steam, such as aniline. In this technique steam is passed through the impure liquid, and the system boils when the sum of the vapour pressure of the organic substance and the vapour pressure of water (aqueous tension) equals the external atmospheric pressure. Mathematically, p_total = p_water + p_organic. Here the total pressure is 760 mm Hg and the aqueous tension contributed by water is 717 mm Hg, so the partial pressure of aniline vapour is 760 - 717 = 43 mm Hg. Because aniline contributes its own vapour pressure, the mixture boils below 373 K, allowing aniline to distil over without reaching its high normal boiling point where it could decompose. The option 717 mm Hg is merely the aqueous tension, not the organic partial pressure. The option 760 mm Hg is the total pressure, not aniline's share. The value 1477 mm Hg wrongly adds the two pressures instead of subtracting. This additive-pressure principle is the NCERT basis of steam distillation. Plausibility check: the organic partial pressure must be smaller than the total, and 43 mm Hg satisfies this.