pH can be caluclated using the following equation: pH = -log[H+(aq)]
Strong Acids: As strong acids fully dissociate, we can assume that the concentration of acid put into the solution is the same as the concentration of hydrogen ions. Calculate the pH of a 0.001mol dm^-3 solution of hydrochloric acid:
[H+] = 0.001 so pH = -log 0.001 = 3.00
Weak Acids: The concentration of the acid and the Ka are needed to calculate the pH of weak acids as only some of the acid as dissociated and so the concentration of hydrogen ions and of the acid put into the solution will be different. Calculate the pH of 0.01mol dm^-3 ethanoic acid (Ka = 0.000017)
0.000017 x 0.01 = 1.7 x 10^-7
√1.7 x 10^-7 = 4.1 x 10^-4 = [H+]
Thus pH = -log(4.1 x 10^-4) = 3.4
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