So far we have expressed equilibrium constant of the reactions in terms of molar concentration of the reactants and products, and used symbol, *K**c *for it.

For reactions involving gases, however, it is usually more convenient to express the equilibrium constant in terms of partial pressure.

The ideal gas equation is written as, pV = nRT

Here, p is the pressure in Pa, n is the number of moles of the gas, V is the volume in m3 and T is the temperature in Kelvin

Therefore, *n*/*V *is concentration expressed in mol/m^{3}

If concentration c, is in mol/L or mol/*d*m^{3}, and *p *is in bar then p = cRT

We can also write *p *= [gas]R*T*

Here, R= 0.0831 bar litre/mol K

At constant temperature, the pressure of the gas is proportional to its concentration i.e.,

*p *∞[gas]

For reaction in equilibrium H2(g) + I2(g) ⇔ 2HI(g)

We can write either

where ∆*n *= (number of moles of gaseous products) – (number of moles of gaseous reactants) in the balanced chemical equation. It is necessary that while calculating the value of *K**p*, pressure should be expressed in bar because standard state for pressure is 1 bar.

We know from Unit 1 that:

1pascal, Pa=1Nm^{-2}, and 1bar = 10^{5} Pa