How to calculate pH of strong acid solutions?
Examples on strong acids include hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4). These acids ionize in solution as described in the following formulas:
Sulfuric acid is diprotic. Its ionization involves a second step. On the other hand, hydrochloric and nitric acids completely ionize in a single step to produce hydronium ions (H3O+). Each mole of the acid produces 1 mole H3O+ .Therefore, a solution with 0.01 molar (mole/ liter) HCl contains 0.01 molar H3O+. The pH can be calculated using the following equation:
Where, [H3O+] is the hydronium ion concentration.
For example, if we have 0.001 M (molar) hydrochloric acid (HCl) solution then
pH = -log (0.001) plugging that into a calculator gives a pH = 3.
If the concentration of the acid is less than 0.00001 M then another equation has to be used in which the ionization of water becomes a factor. For example, if the concentration of the acid is 0.0000001 M (1×10-7 M) then applying the above equation gives a value of pH=7! which is not correct to have a neutral pH after adding an acid to solution even if the concentration is so small. Moreover, if the concentration of the acid is 0.00000001 M (1×10-8 M) then applying the above equation gives a pH=8!! which is not correct at all to have a pH in the alkaline region after adding some acid.
Examples on strong acids include hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4). These acids ionize in solution as described in the following formulas:
HCl
+ H2O → H3O+ + Cl-
HNO3
+ H2O → H3O+ + NO3-
H2SO4
+ H2O → H3O+ + HSO4-
Sulfuric acid is diprotic. Its ionization involves a second step. On the other hand, hydrochloric and nitric acids completely ionize in a single step to produce hydronium ions (H3O+). Each mole of the acid produces 1 mole H3O+ .Therefore, a solution with 0.01 molar (mole/ liter) HCl contains 0.01 molar H3O+. The pH can be calculated using the following equation:
pH = -log [H3O+]
Where, [H3O+] is the hydronium ion concentration.
For example, if we have 0.001 M (molar) hydrochloric acid (HCl) solution then
pH = -log (0.001) plugging that into a calculator gives a pH = 3.
If the concentration of the acid is less than 0.00001 M then another equation has to be used in which the ionization of water becomes a factor. For example, if the concentration of the acid is 0.0000001 M (1×10-7 M) then applying the above equation gives a value of pH=7! which is not correct to have a neutral pH after adding an acid to solution even if the concentration is so small. Moreover, if the concentration of the acid is 0.00000001 M (1×10-8 M) then applying the above equation gives a pH=8!! which is not correct at all to have a pH in the alkaline region after adding some acid.
Grrateful for sharing this
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