A solution has a pH =9, it is 1000 times more basic than the original solution. What was the pH of the original solution?
 

pH=−log$\left[{\mathrm{H}}^{+}\right]$ = 9
Hence [H⁺]=10^−9.
When solution is one thousand as basic as a solution of pH, hydrogen ion concentration is increased 1000 times,
[H⁺]=10⁻⁶
pH=−log$\left[{\mathrm{H}}^{+}\right]$=−log10⁻⁶=6.