Verified Answer :

TSO

pH = -log([H+])
[H+] = 10^(-pH)

[H+] = 10^(-8.78) = 1.6510^-9

[H+][OH-] = Kw
Kw = 1.0
10^-14 at 25 degrees celsius.

[OH-] = Kw/[H+] = (1.010^-14)/(1.6510^-9) = 6.0610^-6

The concentration of OH- ions is 6.1
10^-6 M.