A barometer tube contains a mixture of air and saturated water vopour in the space above the mercury column. It reads 70cm when the actual atmospheric pressure is 76cm of mercury. The saturation vapour pressure at roon tempature is1.0 cm of mercury, The tube is now lowered in the reservoir till the space above the mercury cloumn is reduced to half its origina volume. Find the reading of the barometer. Assume that the temperature remains constant.

Explanation

The pressure due to the air +vapour is `76cm-70cm=6cm` of mercury, The vapour is saturted and the pressure due to it is 1cm of mercury. The pressure due to the air is, therefore, 5cm of mercury,
As the tube is lowered and the volume above the mercury is decreased, Some of the vapour will condense. The remaining vapour will again exert a pressure of 1 cm of mercury . the pressure due to air is doubled as the volume is halved. Thus, pair`=2xx5cm=10cm` of mercury. The pressure due to air=vapour `=10cm+1cm=11cm`mercury. The barometer reading is `76cm-11cm=65cm.`