1. The pH Scale & pH Calculations Chemistry 10 Mrs. Page

2. Learning Objectives: Explain and use the pH scale Use a pH meter and universal indicators to determine the acidity or alkalinity of a substance Solve problems involving pH and [H + ]

3. The pH Scale pH is a measure of how acidic or basic a solution is, which means it is a measure of the concentration of hydrogen ions [H + ] in a solution. The pH scale ranges from 0 to 14. Acidic solutions have pH values below 7 A solution with a pH of 7 is neutral (pure water) Basic/alkali solutions have pH values above 7. Explain and use the pH scale

4. pH Scale The pH scale is a way of expressing the amount of dissociation of acids and bases in water. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H + (or OH - ) ions. Explain and use the pH scale

5. pH Scale The pH scale is a logarithmic scale (not linear). A change of 1 pH unit represents a tenfold change in the acidity of the solution. For example, if one solution has a pH of 1 and a second solution has a pH of 2, the first solution is not twice as acidic as the second—it is ten times more acidic. Explain and use the pH scale

6. Measuring pH Indicators are substances that turn different colors in the presence of an acid or base Universal Indicator – color indicates pH Litmus paper – turns red acid, turns blue base Use a pH meter and universal indicators to determine the acidity or alkalinity of a substance

7. Measuring pH Indicators are substances that turn different colors in the presence of an acid or base Phenolphthalein: commonly used in titrations, turns pink in basic solutions Methyl orange: commonly used in titrations, turns orange in acidic solutions Use a pH meter and universal indicators to determine the acidity or alkalinity of a substance

8. Measuring pH pH Sensor: digital tool for measuring pH Must be used with a GoPro and Logger Pro software Gives reading as a pH number Use a pH meter and universal indicators to determine the acidity or alkalinity of a substance

9. pH Calculations Solve problems involving pH and [H + ] pH = - log [H+] (Remember that the [ ] mean Molarity) Calculate the pH of a solution if the [H + ] is 1 X 10 -10 pH = -log [H+] = -log (1 x 10 -10 ) pH = 10 CAREFUL: on you calculator use (-) for negative & use () around numbers

10. pH Calculations Solve problems involving pH and [H + ] pH = - log [H+] (Remember that the [ ] mean Molarity) Calculate the pH of a solution if the [H + ] is 1.8 X 10 -5 pH = -log [H+] = -log (1.8 x 10 -5 ) pH = 4.7

11. pH Calculations Solve problems involving pH and [H + ] pH = - log [H+] (Remember that the [ ] mean Molarity) Find the pH of a 0.15 M solution of Hydrochloric acid pH = -log [H+] = -log (0.15) pH = 0.82

12. pH Calculations Solve problems involving pH and [H + ] pH = - log [H+] (Remember that the [ ] mean Molarity) Find the pH of a 3.00 X 10 -7 M solution of Nitric acid pH = -log [H+] = -log (3.00 x 10 -7 ) pH = 6.52

13. pH Calculations Solve problems involving pH and [H + ] pH = - log [H+] If you have pH you can also calculate the [H+] -pH = log [H+] Take antilog (10 x ) of both sides and get 10 -pH = [H+] * to find antilog on your calculator, look for “Shift” or “2 nd function” and then the log button

14. pH Calculations Solve problems involving pH and [H + ] If the pH of Coke is 3.12, what is the [H+] ? 10 -pH = [H+] 10 -3.12 = [H+] [H+] = 7.59 x 10 -4 M

15. pH Calculations Solve problems involving pH and [H + ] A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? 10 -pH = [H+] 10 -13.4 = [H+] [H+] = 3.2 x 10 -9 M

16. pH Calculations Solve problems involving pH and [H + ] A solution has a pH of 1.5. What is the Molarity of hydrogen ions in the solution? 10 -pH = [H+] 10 -8.5 = [H+] [H+] = 0.032M