Questions:
How many moles of $\ce{NaOH}$ are needed to change $500~\mathrm{L}$ of solution with $\mathrm{pH}\ 2$ to $\mathrm{pH}\ 11$?
A swimming pool contains 2 million litres of water at $\mathrm{pH}\ 7.80$ and the $\mathrm{pH}$ needs to be changed to $6.80$. How many moles of $\ce{HCl}$ are needed?
Not quite sure how to tackle these types of questions.
Would I have to find the moles of hydroxide/hydrogen ions to neutralise the solution first (ie. moles of hydroxide ions needed to reach $\mathrm{pH}\ 7$) and then from there calculate the amount of hydrogen/hydroxide ions needed to get to final $\mathrm{pH}$?
Or do I just calculate the difference between the moles of ions in final $\mathrm{pH}$ with moles of ions in initial $\mathrm{pH}$?
I have tried both methods but I don't have the answer to the questions so I'm just trying to understand the concept.