3. Thursday, May 1 st : “A” Day Friday, May 2 nd : “B” Day Agenda  Homework questions/collect  Finish section 15.2  Section 15.2 workday: Practice pg. 545: 1,2,4 Sec. 15.2 review, pg. 547: #1-12 Next time: Quiz over section 15.2

4. Homework Questions? Practice pg. 541: 1,2,4,5 Practice pg. 544: 1-4

5. Measuring pH  Because of the negative sign, as the hydronium ion concentration increases, the pH will decrease.  A solution of pH 0 is very acidic.  A solution of pH 14 is very basic (alkaline).  A solution of pH 7 is neutral.

6. pH values of some common materials

7. Indicators  Certain dyes, known as indicators, turn different colors in solutions of different pH.  An indicator is a compound that can reversibly change color depending on the pH of the solution or other chemical change.

8. pH Meters  A pH meter is an electronic instrument equipped with a probe that can be dipped into a solution.  The probe has two electrodes, one of which is sensitive to the hydronium ion.  An electrical voltage develops between the two electrodes, and the circuitry measures this voltage.  The instrument converts the measurement into a pH reading, which is displayed on the meter’s screen.

9. Indicators vs. pH Meters  Indicators Quick and convenient Do not give very precise results  pH Meters Very precise More complicated and expensive or

10. Measuring pH  We know how to calculate the pH from the [H 3 O + ] or [OH - ], but can we calculate the [H 3 O + ] or [OH - ], if we know the pH? You bet your buttons we can!

11. Measuring pH  The pH equation may be rearranged to calculate the hydronium ion (H 3 O + ) concentration from the pH. [H 3 O + ] = 10 −pH

12. Sample Problem C, pg. 545 Calculating [H 3 O + ] and [OH - ] from pH  What are the concentrations of the hydronium and hydroxide ions in a sample of rain water that has a pH of 5.05? [H 3 O + ] = 10 −pH [H 3 O + ] = 10 -5.05 [H 3 O + ] = 8.91 X 10 -6 M K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14 (8.91 X 10 -6 M) [OH-] = 1.00 X 10 -14 [OH - ] = 1.12 X 10 -9 M (3 sig figs)

13. Additional Example  If the pH of a solution is 8.1, what is [H 3 O + ] in the solution? What is the [OH - ] in the solution? [H 3 O + ] = 10 −pH [H 3 O + ] = 10 –8.1 [H 3 O + ] = 7.9 X 10 -9 M K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14 (7.9 X 10 -9 ) [OH - ] = 1.00 X 10 -14 [OH - ] = 1.3 X 10 -6 M (2 sig figs)

14. Section 15.2 Workday  Practice pg. 545: #1,2,4  Section 15.2 review: Pg. 547: #1-12  Section 15.2 concept review: “Acidity, Basicity, and pH” Next time… Quiz over section 15.2