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Understanding the pH Scale and pH Calculations in Chemistry

Explore the autoionization of water, pH scale principles, calculations, and common substances pH review in chemistry. Learn how to calculate pH and solve for hydrogen ion concentration in various solutions.

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Understanding the pH Scale and pH Calculations in Chemistry

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  1. Chapter 19 The pH Scale

  2. H2O(l) + H2O(l) H3O+(aq) + OH−(aq) Autoionization of Water • As we have seen, water is amphoteric. • In pure water, a few molecules act as bases and a few act as acids. • It breaks apart into + an – part • This is referred to as autoionization.

  3. I. Calculating pH -log [H+] or –log [H3O+] A. pH= B. pOH= -log [OH-] 14 C. pH + pOH = Neutral pH 7 14 0 Acidic Basic  pOH 0 14

  4. A. The pH Scale • Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

  5. Review

  6. pH of Common Substances

  7. Calculating the pH pH = - log [H+] (Remember that the [ ] mean Molarity) Example: If [H+] = 1 X 10-10pH = - log 1 X 10-10 pH = - (- 10) pH = 10 Example: If [H+] = 1.8 X 10-5pH = - log 1.8 X 10-5 pH = - (- 4.74) pH = 4.74

  8. pH calculations – Solving for H+ If the pH of Coke is 3.12, [H+] = ??? Because pH = - log [H+] then - pH = log [H+] Take antilog (10x) of both sides and get 10-pH =[H+] [H+] = 10-3.12 = 7.6 x 10-4 M *** to find antilog on your calculator, look for “Shift” or “2nd function” and then the log button

  9. pH calculations – Solving for H+ • A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H+] 8.5 = - log [H+] -8.5 = log [H+] Antilog -8.5 = antilog (log [H+]) 10-8.5 = [H+] 3.16 X 10-9 = [H+]

  10. The pH of a Solution • pH = log[H3O+] and [H3O+] = 10pH • Acidic pH < 7.00 • Neutral pH = 7.00 • Basic pH > 7.00 E.g. determine the pH of a solution in which [H3O+] = 5.40x106 M E.g.2 determine the pH of a solution in which the [OH] = 3.33x103 M E.g.3 determine the pOH of a solution in which the [OH] = 3.33x103 M E.g.4 Determine the [H3O+] if the pH of the solution is 7.35. • The term pX is defined in exactly the same way as pH. Eg.5 What is the pCa if [Ca2+] = 6.44x10-4

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