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A. Calculation of pH of strong monoprotic acids
(Any acids that have a very large Ka value e.g. HCl, HNO3, H2SO4)
The main point is that strong means 100% ionized. Thus, the [H+] of a strong acid is equal to the concentration of the acid.
We assume that virtually all of the acid dissociates thus, virtually no acid molecules remain in solution.
(Significant figures for logarithms: The rule is that the number of decimal places in the log is equal to the number of significant figures in the original number)
Problem 1: Calculate the pH of a 0.010 M solution of HCl.
Solution:
Since HCl is a strong acid it is 100% ionized.
Therefore, [H+] = 0.010 M
pH = - log (0.010) = 2.00 (Note: two significant figures in 0.010, therefore, two decimal places in the log.)
Problem 2. Calculate the pH of a 1.0 M solution of HNO3.
Solution:
Since HNO3 is a strong acid it is 100% ionized.
Therefore, [H+] = 1.0 M
pH = - log (1.0) = 0.00
Problem 3. Calculate the pH of a 5.0 M solution of HCl.
Solution:
Since HCl is a strong acid it is 100% ionized.
Therefore, [H+] = 5.0 M
pH = - log (5.0) = - 0.70
Link to ChemTeam for calculation of the pH of Strong Acids and Strong Bases
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