A barometer correctly reads the atmospheric pressure as 76 cm of mercury. Water droplets are slowly introduced into the barometer tube by a dropper. The height of the mercury column first decreases and then becomes constant. If the saturation vapor pressure at the atmospheric temperature is 0.80 cm of mercury, find the height of the mercury column when it reaches its minimum value.

Given:


Atmospheric pressure=76cm of Hg


Saturation vapor pressure=0.80 cm of Hg


When the water is introduced in barometer, water evaporates.


Thus, it exerts its vapor pressure over the mercury meniscus.


As more and more water evaporates, the vapor pressure increases that forces down the mercury level further down.


Finally, when the volume is saturated with the vapor at atmospheric temperature, the highest vapor pressure, i.e. saturation vapor pressure is observed, and the fall of mercury level reaches its minimum.


Thus,


Net pressure acting on the column = atmospheric pressure -saturation vapor pressure


= 76-0.80 =75.2 cm of Hg.


The height of the mercury column when it reaches its minimum value is 75.2cm.


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