1. Ch. 18: Acids & Bases Sec. 18.3: What is pH?

2. Objectives n Explain the meaning of pH and pOH. n Relate pH and pOH to the ion product constant for water. n Calculate the pH and pOH of aqueous solutions.

3. Ion Product Constant for Water n Recall.. Pure water contains equal amounts of H + and OH - because of the self-ionization of water: H 2 O H + + OH - H 2 O H + + OH - n The H + ][OH - ] n The ion product constant for water (K w ) is [H + ][OH - ] n At 298 K, the concentrations have been determined to be 1 x 10 -7 M. n Therefore, 10 -14 M 2. n Therefore, K w = 1 x 10 -14 M 2.

4. Ion Product Constant for Water n H + ][OH - ] always equals 10 -14 M 2 at 298 K. n [H + ][OH - ] always equals 1 x 10 -14 M 2 at 298 K. n This means, if the H + ] increases, then the [OH - ] must decrease (and vice versa.) n This means, if the [H + ] increases, then the [OH - ] must decrease (and vice versa.)

5. Practice Problems n The H + ] of an aqueous solution is 10 -5 M. What is the [OH - ]? Is the solution acidic, basic or neutral? n The [H + ] of an aqueous solution is 1 x 10 -5 M. What is the [OH - ]? Is the solution acidic, basic or neutral? n If the [OH - ] of an aqueous solution is 10 -3 M, what is the H + ]? Is the solution acidic, basic, or neutral? n If the [OH - ] of an aqueous solution is 1 x 10 -3 M, what is the [H + ]? Is the solution acidic, basic, or neutral?

6. pH n S.P.L. Sorenson developed the pH scale in 1909. pH stands for “power of hydrogen”. n [H + ] is not expressed as a small number in scientific notation but as a logarithm.

7. pH n [H + ] or - log [H + ]. n pH is defined as the negative logarithm of the [H + ] or - log [H + ]. n Think of it this way: the pH of a solution is the negative of the exponent on 10 that gives us the [H + ]. n Given a solution that has [H + ] = 1 x 10 -3 M, its pH is -(the exponent on 10) which is -(-3) or 3.

8. pH This means... n Acids have a pH from 0 to below 7. The lower the number, the stronger the acid. n Neutral (not acidic or basic) is at 7. n Bases have a pH from above 7 to 14. The higher the number, the stronger the base.

9. Practice Problems Calculate the pH of solutions having the following ion concentrations. 1 [H + ] = 1 x 10 -6 M 2 [H + ] = 1 x 10 -7 M 3 [OH - ] = 1 x 10 -5 M Are these solution acidic, basic, or neutral?

10. pOH n is the negative logarithm of the [OH - ] or -log [OH - ]. n pOH is the negative logarithm of the [OH - ] or -log [OH - ]. n Therefore, a solution with [OH - ] = 1 x 10 -2 M has a pOH of 2. n A simple relationship between pH & pOH makes it simple to calculate one when the other is known: pH + pOH = 14 pH + pOH = 14

11. Practice Problems n If the [OH - ] = 1 x 10 -3 M, what is the pOH and pH of the solution? Is the solution acidic, basic, or neutral? n A solution has a pH of 8. What are the [H + ] & [OH - ]? Is the solution acidic, basic, or neutral? n A solution has a pOH of 4. What are the [H + ] & [OH - ]? Is the solution acidic, basic, or neutral?