The usefulness of the pH scale depends on the case with which you can interconvert pH and hydrogen ion concentration.lets see a solved example:
Sample problem 1 : The pH of arterial human blood is 7.40 .What is hydrogen in concentration?
Step1: the pH = - log [H+]
So, hydrogen ion concentration is [H+] = antilog (-pH) = 10^-pH
Step2 : On some calculators,you obtain the anti logarithm by taking the inverse of the logarithm .On others,you use a 10x key.
Step3: the results is [H+]= antilog(-7.40)= 10^-7.40 = 4.0 x 10-8 M