At constant temperature 80% AB dissociates into A2 and B2, then the equillibrium constant for   is

Let nAB = 100Percentage of decompositions of AB = 802 moles of AB decomposes 1 mole of A2 and 1 mole of B280 moles of AB decomposes 40 moles of A2 and 40 mole of B2