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QPages 34-37: Q50: HC Verma: Concepts of Physics - Volume 2 Kinetic Theory of Gases Pages 34-37: Q50 Solution: A barometer tube is 80 cm long above the mercury reservoir). It …

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often mistake the 80 cm tube length for the vapor pressure. The relative humidity is found from the *drop* in mercury height (0.6 cm Hg), not from the difference between the tube length and the reading. Use the change in column height to compute the vapor pressure, then divide by the saturation pressure to get RH.

A barometer tube is 80 cm long above the mercury reservoir). It reads 76 cm on a particular day. A small amount of water is introduced in the tube and the reading drops to 75.4 cm. Find the relative humidity in the space above the mercury column if the saturation vapour pressure at the room temperature is 1.0 cm.

Step-by-Step Verified Solution

**Answer:** Relative humidity = 60% **Step-by-Step Explanation:** Barometer reading initially = 76 cm. After introducing water, reading = 75.4 cm. The difference is due to the vapour pressure of water:
Pvapour=76−75.4=0.6 cm HgP_{vapour} = 76 - 75.4 = 0.6 \text{ cm Hg}
Saturation vapour pressure at room temperature = 1.0 cm Hg.
RH=0.61.0=0.60=60%RH = \frac{0.6}{1.0} = 0.60 = 60\%

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