1 pH How do we measure the amount of “stuff” in a solution? **concentration** How do we determine a solution’s concentration? **molarity Same way with acids and bases except we look at the hydrogen and hydroxide ions to determine how concentrated (ie acidic or basic) a solution is. How can we measure the acidity or basicity of a solution? How would this measurement relate to the hydrogen and hydroxide ion concentrations in solution?

2 Let’s take a look at water2H2O  H3O+ + OH- [H3O+][OH-]= 1x10-14M2 [H3O+]= 1x10-7M [OH-]= 1x10-7M Kw= 1x10-14M2 When water is neutral (neither acidic or basic), hydrogen and hydroxide ions dissociate and each have a concentration of 1x10-7 mol/L or M. This value is obtained at 25 degrees Celcius and equilibrium. If we combine these concentrations together, the product = 1x10-14M = Kw (water dissociation equilibrium constant) Ions dissociate in water is small compared to when acid or base is added SO we will be looking at concentrations of the acid or base added. We are only going to be dealing with strong acids and bases

3 Example 1 An acid is added to water and gives a hydroxide ion concentration [OH-] of 1.0x10-12M. What is the hydrogen ion concentration [H+] ? What happens to the hydrogen ion concentration when we add acid to water? What about hydroxide ions with base? Example 1: [OH-] = 1.0x10-12M *Write equation, 1.0x10-14M2 = [H+][OH-], plug in what we know and solve. 1.0x10-14M = [H+] (1.0x10-12M), [H+] = 0.01M or 1x10-2M **So this value tells us the concentration of a solution, how acidic it is? Is this value easy to understand? NO, so the pH scale was developed to easily determine how acidic or basic a solution is. how many of you have used a pH meter or pH indicators for a pool, hot tub, etc.? Why do you need to know the pH, what do you do? We have certain pH levels that must be maintained in our body for us to function normally (blood pH, stomach acid pH, etc.). *may mention buffers as a way of ensuring pH levels remain stable*

4 What is pH? Pouvoir hydrogene: “hydrogen power”pH = measure of [H3O+] Acidity [H3O+] expressed in powers of 10 Ex to 10-1

5 pH Scale Range from 0-14. NEUTRAL, pH=7. (pure water)BASE, pH > 7. (ocean water, milk of magnesia, baking soda) ACID, pH < 7. (stomach acid/HCl, vinegar, soft drinks) Logarithmic scale= base 10 system, relationship between a solution’s acidity/basicity and the concentration of H+ ions in solution Ex. [H+] = 1x10-7 is a pH of 7 (take the negative log), neutral. [H+] = 1x10-10 is a pH of 10, basic, more or less hydrogen ions in solution? LESS * draw out number [H+] = 1x10-2 is a pH of 2, acidic, more or less hydrogen ions in solution? MORE *draw out number Blood pH is , mostly neutral. Hydrogen ion concentration is inversely proportional to the scale (pH goes down/increase H ions, pH goes up/decrease H ions)—try with OH ions too. *have students write it down.

6 pH Scale Draw pH scale. Look at common things on scale.

7 How do we measure the pH of a solution?Acid-base indicators (ex. litmus paper) pH meter Ask students if they have measured pH in a pool, etc. What did they use? Once we get the pH of a solution, how can we find the concentration of H ions in a solution? Use the log relationship between pH and H ions Types of indicators: methyl red, phenolphtalein, methyl orange (indicators have a certain pH range for a color change)

8 Equations pH = -log[H+] pOH = -log[OH-] pH + pOH = 14[H+][OH-] = 1x M2 Here are the equations we use based on this relationship. If given a pH, we use the antilog to find the H ion concentration: pH=3 so [H+] = 10-3 *write on board pOH measures the hydroxide ion concentration in a solution, not really dealing with this value much.

9 Example 2: What is the pH of a solution with a hydrogen ion concentration of 1x10-4M ? Is this solution acidic or basic ? What are you trying to find? What equation describes this relationship? WRITE DOWN THE EQUATION!!! pH= -log [H] pH= -log (1.0x10-4) = 4, acidic solution

10 Example 3: What is the hydrogen ion concentration of a solution with a pH of 11 ? Is this solution acidic or basic ? What are you trying to find? What equation describes this relationship? WRITE DOWN THE EQUATION!!! Now we only have pH and asking for H concentration. pH= -log [H] 11 = -log [H], -antilog(11) = [H], [H] = 1x10-11M, basic solution.

11 Example 4: Calculate the [H+] and [OH-] of a vinegar solution with a pH of 2.5.

12 Now, you try…… 1) [H3O+] = 1.5x10-6M. Find the pH.2) A solution’s pH is What are the [H3O+] and [OH-] concentrations in the solution? If given the name of the acid or base, write the dissociation equation. For example, HCl---H+ + Cl- Answers: 1) pH= ) [H3O+]= 1.9x10-4M , [OH-] = 5.3x10-11M

13 Thought Question When an acid is added to a beaker of water, what happens to the [H3O+] and [OH-] in the solution? Why? What happens to the pH? Why? #38 on p. 473 of college Chemistry book.