mam,my question is why does'nt a weak acid dissociate completely?
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogen in a solution, donating only a partial amount of its protons to the solution. Equilibrium exists between the weak acid, water, H3O+, and the anion of the weak acid. The equilibrium lies to the left hand side of the equation, indicating that not much H3O+ is being produced. The fact that very little H3O+ is being produced is the indication of a weak acid. We generally consider any acid that is weaker than the hydronium ion, H3O+ (the strongest acid that can exist in water) does not dissociate fully. For example: If we put CH3COOH, a weak acid, in water, it will exist in equilibrium with the hydronium ion.
CH3COOH + H2O <------> CH3COO- + H3O+
with equilibria like this, we have two acids that are trying to protonate each other's respective conjugate base (the anion of the acid). Because the hydronium ion is a stronger acid than ethanoic acid, it will more easily protonate acetate than ethanoic acid protonates water. Therefore we have the equilibria shifted to left hand side and we have a large amount of undissociated ethanoic acid (our weak acid).