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pH Calculator

The pH of an aqueous solution is calculated using the expression:  pH = -log[H3O+]  The process by which one determines the [H3O+] of a solution depends up the solution and how the solution was made. There are at least 26 different ways to calculate the pH . . . . the key to pH calculation is to determine what is present at equilibrium . . . . strong acid, strong base, weak acid, weak base, or buffer solution. The pH Calculator below guides the student through the construction of ICE Tables, checks their math along the way and ends with a calculation of pH, pOH, [H3O+] and [OH-].

  1. 0.325 M HCl solution (strong acid - 100% ionized)
  2. 10.0 mL of 0.325 M HCl + 4.5 mL H2O (solve as strong acid, but account for the dilution that occurred)
  3. 0.143 M NaOH solution (strong base - 100% ionized)
  4. 10.0 mL of 0.143 M NaOH + 6.7 mL H2O (solve as strong base, but account for the dilution that occurred)
  5. 0.235 M HF solution (weak acid - HF partially dissociates in water . . . . use the pka)
  6. 10.0 mL of 0.235 M HF + 13.2 mL H2O (solve as weak acid, but account for the dilution that occurred)
  7. 0.543 M NaNO2 solution (weak base - NO2- partially reacts with water . . . . use the pkb)
  8. 10.0 mL of 0.543 M NaNO2 + 11.2 mL H2O (solve as weak base, but account for the dilution that occurred)
  9. HCl + NaOH - acid and base react completely until one (or both) is used up . . . .
    • HCl is in excess (solve as if only a strong acid is present - account for the dilution that occurred)
    • NaOH is in excess (solve as if only a strong base is present - account for the dilution that occurred)
    • equimolar amounts of HCl and NaOH are added (pH = 7)
  10. HCl + HF (the weak acid's ionization is significantly muted - consider only the strong acid's concentration and account for the dilution that occurred)
  11. HCl + NaF - acid and base react completely until one (or both) is used up . . . .
    • HCl is added in excess and HCl / HF are present in the solution (solve as if only a strong acid is present - account for the dilution that occurred)
    • F- is added in excess and F - / HF are present in the solution (solve as a buffer solution using Henderson-Hasselbach Equation)
    • equimolar amounts of HCl and NaF are added and only HF is present in the solution (solve as a weak acid solution - account for the dilution that occurred)
  12. NaOH + NaF (the weak base's reaction with water is significantly muted - consider only the strong base's concentration and account for the dilution that occurred)
  13. NaOH + HF - acid and base react completely until one (or both) is used up . . . .
    • NaOH is added in excess and NaOH / F- are present in the solution (solve as if only a strong base is present - account for the dilution that occurred)
    • HF is added in excess and HF / F -are present in the solution (solve as a buffer solution using Henderson-Hasselbach Equation)
    • equimolar amounts of NaOH and HF are added and only F- is present in the solution (solve as a weak base solution - account for the dilution that occurred)
  14. HF + NaF - this is a buffer solution (solve using Henderson-Hasselback Equation)
  15. HF / F- + HCl - this is a buffer solution to which strong acid is added (the HCl reacts with the F- ion to form HF).
    • the moles of HCl < moles F- . . . . both HF and F- are present at equilibrium (solve as a buffer solution using Henderson-Hasselbach Equation).
    • the moles of HCl = moles F- . . . . only HF is present at equilibrium (solve as a weak acid solution - account for the dilution that occurred).
    • the moles of HCl > moles F- . . . . both HCl and HF are present at equilibrium (solve as if only a strong acid is present - account for the dilution that occurred).
  16. HF / F- + NaOH - this is a buffer solution to which strong base is added (the NaOH reacts with the HF to form F- ion).
    • the moles of NaOH < moles HF . . . . both HF and F- are present at equilibrium (solve as a buffer solution using Henderson-Hasselbach Equation).
    • the moles of NaOH = moles HF . . . . only F- is present at equilibrium (solve as a weak base solution - account for the dilution that occurred).
    • the moles of NaOH > moles HF . . . . both NaOH and F- are present at equilibrium (solve as if only a strong base is present - account for the dilution that occurred).

What's the value of the Chem21Labs pH Calculator? Our classroom experiences teaching pH concepts led us to develop this interactive "decision map" - a visual diagram of the logical structure of decision-making steps in solving pH problems. By using the pH Calculator, its logical structure of solving pH problems becomes the mental schema used by students to solve pH problems. The pH Calculator guides students through the many different ways of calculating the pH of a solution and gives students immediate feedback on their calculations. In a pH lab developed by Chem21Labs, the use of this calculator is a required part of the lab . . . . the correct answer is only accepted if the pH Calculator was used. While this initially may seem "rigid", this particular lab has students work in groups (2-4) and  all of their lab answers  are the same. Experience has shown that not all the members of the group build the necessary schema when there exists a "shorter route" to the answer. Without doubt, using the pH Calculator is time consuming . . . . but, only in the sense that it takes longer than copying another student's work . . . . and,  students learn how to solve pH problems . Additionally, the pH Calculator is used as an optional, interactive animation in the Chem21Labs homework pH problems.


Have some fun . . . . use the pH Calculator below to determine the pH, pOH, [H3O+] and [OH-] of a 0.10 M HCl solution.

  1. Select SA (Strong Acid) and click the Continue button (lower right).
  2. Select HCl from the list of Strong Acids.
  3. Enter .10 for the concentration of HCl and click the Continue button.
  4. Enter "C" then "l" (click the Aa button to access lowercase letters) then -.
  5. Click the Check Answer button (lower right) . . . . note there is a to the right of Cl-.
  6. Enter .10 for the "C" row in the ICE table.
  7. Click the 1st Edit button in the "E" row and enter 0.
  8. Click the 2nd Edit button in the "E" row and enter .10.
  9. Click the 2nd Edit button in the "E" row and enter .10.
  10. Click the Check Answer button . . . . note there are four .
  11. Enter 1.0 for the pH.
  12. Click the Check Answer button . . . . note the to the right of the pH.
  13. Click the Continue button.
  14. Enter 1E-13 for the [OH-].
  15. Click the Edit button to the left of pOH and enter 13.0.
  16. Click the Check Answer button . . . . note the two to the right of the correct answers.
  17. When integrated into a lab, this final screen of the pH Calculator has a student-specific number (1-5) in the upper right corner. The student would enter the pH information on this page along with their random number . . . . this ensures that the student was logged in to their account and used the pH Calculator on their webpage to arrive at the answers shown on this page . . . .  Translation:  if a student receives help, the person helping them must go through the steps you just completed to demonstrate how to get the answer.