If it is given as "Same moles of reactants has taken as the initial moles" what should we use as initial moles? is it correct to use like "a" mole each or else take it as 1 mole each? also i have heard if the amount of dissociation given, then take initial mole as 1.if not take it just as "a".but i am kind of confuse.hope you understand. please help me.
While using the degree of dissociation concept it is advantageous to take 1 as the initial number of mole first, then subtract the degree of dissociation, alpha from it and finally multiply the whole thing with the molar concentration of the species, c. You will get the actual equilibrium concentration of the reactants at any given point.
For example, in the following reaction, starting with c molar A and c molar B, if alpha be the degree of dissociation then we can write the reactant concentrations at equilibrium in the following manner:
A+B `stackrel(larr)(->)` C+D
dissociation: alpha alpha
actually (eqm.): c (1-alpha) c (1-alpha)
You can instead, use 'a' as the initial number of moles of reactants and then subtract moles reacted, x from it to arrive at the same target. Therefore, both the methods end up at the same conclusion if applied judiciously to a particular system.