Question

1.Why are there two different values for R?(R = 8.314472 JK^-1mol^-1 = 0.0820574 L atm K^-1 mol^-1

^2.Why are liquid barometers filled with mercury rather than water?

3. Using the concepts of the kinetic theory of gases, describe WHY one gas will diffuse through another.

4. Why does hot air rise? Prove your answer using the ideal gas equation

5. Why do gases show a positive deviation from ideal behaviour at high pressures? (i.e Why don’t they only show negative deviations?)

Answer 1

1. Basically it depends on the units. When we use Volume in Litres and pressure in atmospheres, we use 0.0823 and if we use volume in m3 and pressure in pascals, we use 8.314.

2. Barometer is used to measure atmospheric pressure. It is an inverted tube inserted into the barometric fluid and the liquid rise in the tube gives us the measure of pressure in the atmosphere. Now liquid mercury rises upto a height of around 760 mm as we know at 1 atmoshpere pressures. At the same pressure liquid water rises to a height of around 10.3 metres! That is huge. We would require 10.3 metres height of tube just to measure 1 atm pressure. Also mercury, being a metal doesnt have any significant vapor pressure at 1 atm whereas water does and it creates a difficulty in calculations.

3. KTG states that gas moleules are in random motion and also that they have negligible volumes. So when two gases come in contact, they diffuse through each other because of their continuous random motions and the volumes being negligible, there is very little probability of colliding.

4. When we heat a gas, according to ideal gas law, PV = nRT, as temperature increases the volume of the gas increases reducing its density. When density of hot air reduces, it becomes lighter than colder air and occupies the space above it.

5. At high pressures, the gas becomes less compressible, compressibiltiy factor is greater than 1. Negative deviation needs the gas to become more compressible at high pressures, which is rarely the case.