[ the (H +) of a solution with pH 4.0 is

In this way, pH is determined by hydrogen-ion concentration. In the case of a neutral solution, [H+]=10-7 , which we call a pH of 7. This means, for example, that a hydrogen-ion concentration of a solution with a pH of 4 is 10-4mol/l, meaning it contains 0.0001 mol of hydrogen ions in a solution of 1 liter. I explained everything I hope it is enough