What is the pH of 1 M HCl?

The pH of 1 M hydrochloric acid is zero. The reason is that pH is defined as negative 1 times the log base 10 of [H+], or the proton concentration in moles per liter. Stated another way, 10 raised to the 0 power equals 1.

Pure water has a proton concentration of 10 ^-7 M, and this corresponds to a pH of 7. As a solution becomes more acidic, its pH lowers. A solution with a pH of 6 has 10 times as many protons as a neutral solution at pH 7. A highly acidic solution such as battery acid can have a pH of zero, which means it has 10 million times as many protons as a sample of pure water.