1. When adding strong base to a buffer, why can we assume the equilibrium value of the OH- is zero?

When adding strong base to a buffer, we assume the equilibrium value of the hydroxyl ion (OH-) is zero because all hydroxyl ion (OH) react with hydrogen ion (H+) forming water. So there is no hydroxyl ion (OH-) left and values of hydroxyl ion (OH-) is become zero.

Hydrogen ion is released by weak acid which is present in the buffer solution.