manometric method

1. Manometric Method
1
 The liquid whose vapour pressure is to be determined is taken in flask
placed in the thermostat. One end of the tube from flask is connected to
manometer and other end is connected to vacuum pump.
 The air above the liquid is removed in the following steps.
 Freezing: The liquid is frozen with the help of freezing mixture and
space above the liquid is evacuated. In this way, air is removed from the
surface of liquid along with the vapours of liquid.
 Melting: The frozen liquid is melted to release entrapped air.
 Re-freezing: Liquid is again frozen and released air is evacuated. This
process is repeated many times till almost all the air is removed.

2. Measurement of vapour pressure
2
 The liquid in the flask is warmed in thermostat at a temperature at which vapour
pressure is to be determined. Difference in the heights of Hg column in two limbs
of manometer determines the vapour pressure of liquid.
 The pressure on the surface of liquid is equal to sum of atmospheric pressure and
vapour pressure of liquid. That is the reason why column of manometer facing the
liquid is more depressed than facing the atmosphere.
The vapour pressure of liquid is given by following
equation.
P = Pa + h
P = vapour pressure of liquid at one atmosphere.
Pa = atmospheric pressure
h = difference in the heights of Hg column in two limbs
of manometer. It gives us the vapour pressure of liquid.