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Concepts Of Physics MCQ Edition [Volume 2]PhysicsKinetic Theory of Gases

A barometer tube has a length of 80 cm (above the mercury reservoir). On a particular day, it reads 76 cm . A small quantity of water is introduced into the tube, causing the reading to drop to 75.4 cm . Determine the relative humidity in the space above the mercury column, given that the saturation vapour pressure at room temperature is 1.0 cm .

Options

  1. A60 %
  2. B80 %
  3. C40 %
  4. D75 %

Correct answer

A. 60 %

Step-by-step solution

Initial reading of the barometer, h₁ = 76 cm . This implies the atmospheric pressure is P_ atm = 76 cm of Hg , and the space above the mercury column is initially a vacuum (pressure = 0 ). When a small quantity of water is introduced, it evaporates and exerts a vapour pressure P_v in the space above the mercury. The new reading of the barometer is given as h₂ = 75.4 cm . Equating the pressures at the base of the tube: P_ atm = h₂ + P_v 76 = 75.4 + P_v P_v = 76 - 75.4 = 0.6 cm of Hg The actual vapour pressure of wat

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