pH of a solution can be calculated using the following equation
pH = -log[H+]
Therefore when the pH is given, to find the H+ ion concentration we have to find out the anti log
To find the reverse we can use the following equation
H+ = antilog [-pH]
We can find the antilog by value of 10^(-6.6)
[H+ ] = 2.5 x 10^(-7) m