The acidity arises from the presence of hydrogen ions (H+) in water solutions. pH is the logarithm scale that quantifies the solution acidity level; pH = - log [H+] where [H+] represents the concentration of hydrogen ions The neutral solution has a pH of 7. Acidic solutions have pH values below 7, while a pH greater than 7 is basic. By definition, a strong acid completely dissociates in the water. It permits the straightforward calculation of pH from the acid concentration.
Write down the acid dissociation reaction. For example, for hydrochloric acid (HCL) the equation is HCl = H(+) + Cl(-).
Analyze the reaction to find out how many hydrogen ions (H+) are produced by the dissociation of the acid. In the example, one molecule of HCl produces one hydrogen ion.
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Multiply the acid concentration by the number of hydrogen ions produced to compute the concentration [H+]. For example, if the concentration of HCL in the solution is 0.02 molar, then the concentration of hydrogen ions is 0.02 x 1 = 0.02 molar.
Take logarithm of the hydrogen ion concentration and then multiply the result by -1 to calculate pH. In the example, log (0.02) = -1.7 and pH is 1.7.