Why can we ignore the weak conjugate base/acid when calculating the pH in a titration problem? [closed]








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Take the reaction 350 mL of 0.8M HCN and 0.6M NaOH for example. The problem asks us the calculate the pH when we add 500 mL of 0.6M NaOH into the 350 mL of 0.8M HCN (This is more than enough to neutralize the weak acid). However, the problem states that we can ignore OH contributions from water or any weak base. Can someone explain why we can do this? 




$\ce{NaOH}$ is a strong base. It is assumed that it **dissociates completely** in aqueous solutions, hence, it's contribution to the $\ce{OH-}$ concentration is the *maximum* as compared to other weak bases or water. Moreover, due to common ion effect, the presence of $\ce{OH-}$ ions in solution due to $\ce{NaOH}$ further hinders the dissociation of the *already* weak bases, thereby making their contributions negligible.


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