. Ionisation of weak acids We have already learnt that weak acids are partially dissociated in water and there is an equilibrium between the undissociated acid and its dissociated ions. Consider the ionisation of a weak monobasic acid HA in water. HA+H O H O +A Applying law of chemical equilibrium, the equilibrium constant K c is given by the expression K = [H O ][A ] [HA][H O] .....( .
) The square brackets, as usual, represent the concentrations of the respective species in moles per litre. In dilute solutions, water is present in large excess and hence, its concentration may be taken as constant say K. Further H O + indicates that hydrogen ion is hydrated, for simplicity it may be replaced by H + . The above equation may then be written as, K = [H ][A ] [HA] K .....( .
) The product of the two constants K C and K gives another constant. Let it be K a K = [H ][A ] [HA] a .....( . ) The constant K a is called dissociation constant of the acid. Like other equilibrium constants, a K also varies only with temperature.
Similarly, for a weak base, the dissociation constant can be written as below. K = [B ][OH ] [BOH] b ....( . ) . .
Ostwald’s dilution law Ostwald’s dilution law relates the dissociation constant of the weak acid (K ) a with its degree of dissociation ( ) α and the concentration (c). Degree of dissociation ( ) α is the fraction of the total number of moles of a substance that dissociates at equilibrium. XII U8-Ionic XII U8-Ionic - - - - Number of moles dissociated total number of moles α We shall derive an expression for ostwald's law by considering a weak acid, i.e. acetic acid (CH COOH) .
The dissociation of acetic acid can be represented as CH COOH H + CH COO The dissociation constant of acetic acid is, k = [H ][CH COO ] [CH COOH] a .....( . ) CH COOH CH COO Initial number of