To calculate the pH of a 0.1 M hydrochloric acid HCl solution, we can use the formula:pH = -log10[H+]Hydrochloric acid is a strong acid, meaning it dissociates completely in water to form H+ and Cl- ions. In this case, the concentration of H+ ions is equal to the concentration of the HCl solution, which is 0.1 M.So, we can plug this value into the formula:pH = -log10 0.1 pH = - -1 pH = 1Therefore, the pH of a 0.1 M hydrochloric acid solution when measured with a pH meter at 25C is 1.