when conc. of strong acid is very low we need to consider common ion effect of [H+] COMMING From water...Now try solving it
When the concentration of a strong acid is lower than 10^-6 we consider the H+ ions present in water also.
So, the conc of H+ now is=10^-6+10^-7....now it is the same old problem! (10^-7 only at 25 degrees celsius)