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 room temperature is `1.0 cm` of mercury, The tube is now lowered in the reservoir till the space above the mercury column is reduced to half its original volume. Find the reading of the barometer. Assume that the temperature remains constant.

Explanation

"The pressure due to the `air + vapour` is `76cm-70cm= 6m` of mercury the vapour is saturated 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 vopour will again exert a pressure of the volme which is halved. Thus" ,
`P_(air)=2xxcm=10cm`"of merucry . The pressure due to the `air+vapout`reading is" `76cm-11cm=65cm`