Chemistry · Solutions · chapter 1 of 10

Class 12 Chemistry Chapter 1: Solutions case study question with answers

One case-based question of the kind the board asks: a passage, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

Ritika, a diving instructor in the Andaman Islands, explains to her trainees why their tanks are not filled with plain air. At a depth of 30 m the total pressure on a diver is about 4.0 bar, and the gas breathed is at this pressure. Gases dissolve in blood roughly as they do in water, following Henry's law. She writes on the board the Henry's law constants in water at 293 K: N₂ 76.48 kbar, O₂ 34.86 kbar, He 144.97 kbar. Her tanks hold 11.7% helium, 56.2% nitrogen and 32.1% oxygen. She also tells them that a trainee who later trekked in Ladakh felt tired, short of breath and unable to think clearly at high altitude.

Part 1 · 1 mark

If plain air (79% N₂) were breathed at 4.0 bar, the mole fraction of N₂ dissolved in water at 293 K would be about

(a) 4.1 × 10⁻⁵(b) 5.2 × 10⁻⁵(c) 2.4 × 10⁴(d) 4.1 × 10⁻²

Show answer

Answer: (a) 4.1 × 10⁻⁵

p(N₂) = 0.79 × 4.0 = 3.16 bar; x = p/K_H = 3.16/76 480 = 4.1 × 10⁻⁵. 5.2 × 10⁻⁵ uses the total pressure; 4.1 × 10⁻² leaves K_H in kbar.

Part 2 · 1 mark

Of the three gases on the board, the one least soluble in water at the same partial pressure is

(a) helium(b) nitrogen(c) oxygen(d) all three equally

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Answer: (a) helium

The higher K_H, the lower the solubility at a given pressure; He has the highest K_H, 144.97 kbar.

Part 3 · 2 marks

Explain why a diver who rises quickly on plain air may suffer the bends, and why helium is used in the tanks.

Answer and marking scheme

At depth the high pressure dissolves more nitrogen in the blood. As the diver rises the pressure falls, the solubility falls, and nitrogen is released as bubbles that block capillaries: the bends. Helium is far less soluble (highest K_H), so diluting the air with helium reduces the gas dissolved and avoids the bends and nitrogen toxicity.

high pressure dissolves more N₂; on rising it comes out as bubbles blocking capillaries (1)
helium has low solubility (high K_H), so less gas dissolves (1)

Explain the trainee's tiredness and confusion in Ladakh using Henry's law, and name the condition.

Answer and marking scheme

At high altitude the partial pressure of oxygen is lower. By Henry's law the solubility of oxygen is proportional to its partial pressure, so less oxygen dissolves in the blood and tissues; the person tires easily and cannot think clearly. The condition is anoxia.

lower partial pressure of O₂ means less O₂ dissolves in blood (Henry's law) (1)
the condition is anoxia (1)

Chapter 2: Electrochemistry →