Question

For an atmospheric pressure of 101 kPa (abs) determine the heights of the fluid columns in

barometers containing one of the following liquids a) mercury, b) water, c) ethyl alcohol.

Calculate the heights including the effect of vapour pressure and compare the results with those obtained
neglecting vapour pressure. Do these results support the widespread use of a particular fluid for
barometers? Explain the reason.

Answer 1

a) 0.759 m , 0.759 m

b) 10.1 m , 10.3 m

c) 12.3 m , 13 m

Step-by-step explanation:

Vapour Pressure included:

`P_(atm,vp) = y*h + P_(vp) \\\\h = (P_(atm,vp)-P_(vp))/(y) ... Eq1`

mercury

`h_(Hg,vp) = (101*10^3-1.6*10^(-1))/(133*10^3) \\\\h_(Hg,vp) = 0.759m`

`h_(Hg) = (101*10^3)/(133*10^3) \\\\h_(Hg) = 0.759m`

water

`h_(H2O,vp) = (101*10^3-1.77*10^3)/(9.8*10^3) \\\\h_(H2O,vp) = 10.1m`

`h_(H2O) = (101*10^3)/(9.8*10^3) \\\\h_(H20) = 10.3m`

Ethyl Alcohol

`h_(EA,vp) = (101*10^3-5.9*10^3)/(7.74*10^3) \\\\h_(EA,vp) = 12.3m`

`h_(EA) = (101*10^3-5)/(7.74*10^3) \\\\h_(EA) = 13m`

For mercury barometers the effects of vapour pressure are negligible and required height for mercury barometer is reasonable as compared to that of water and ethyl alcohol.