Question

Which weak acid would be best to ude when preparing a buffer solution with ph of 9 in .in?

Answer 1

buffer solution is formed by mixing weak acid with a strong base or strong acid with weak base.

In above case buffer has to Ph -9 its basic buffer so need to use

ortho Phosphoric acid (H3PO4)

or acid which Ka value  near about  1 * 10^-9

I hope it will be clear