A barometer tube contains a mixture of air and saturated water vapour in the space above the mercury — States of Matter and Gaseous State Chemistry Question
Question
A barometer tube contains a mixture of air and saturated water vapour in the space above the mercury column. If reads 70 cm when the actual atmospheric pressure is 76 cm of Hg. The saturation vapour pressure at room temperature is 1.0 cm of Hg. The tube is now lowered in the reservoir till the space above the mercury column is reduced to half of its original volume. The reading of barometer (in cm) is (Assume that the temperature remains constant.)
💡 Solution & Explanation
Initial reading is 70 cm, atmospheric is 76 cm. Total pressure inside space $= 76 - 70 = 6\text{ cm Hg}$. Saturation vapor pressure $= 1.0\text{ cm Hg}$. Pressure of dry air $= 6 - 1 = 5\text{ cm Hg}$. When volume is halved, pressure of dry air doubles to $2 \times 5 = 10\text{ cm Hg}$. Saturated vapor pressure remains constant at $1.0\text{ cm Hg}$ (some condenses). Final total pressure inside space $= 10 + 1 = 11\text{ cm Hg}$. Final reading of barometer $= 76 - 11 = 65\text{ cm Hg}$. Therefore, correct answer is 0065.