Why we use a water

When reacting an acid or a base with pure water, why do we use the quadratic formula? But when we react say a base with a strong acid, we use Henderson hasselbach equation? What's the concept/reasoning behind it

1. during the synthesis of imine experiment, we use few ethanol for suction filteration. if we use excess of ethanol, the inverse reaction will be occured. explain the reaction with mechanism.