1 The pH Scale The Power or Potential of Hydrogen

2 pH scale the pH scale is a way of expressing the strength of acids and basesthe pH scale is a way of expressing the strength of acids and bases instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H + ioninstead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H + ion under 7 = acid 7 = neutral over 7 = baseunder 7 = acid 7 = neutral over 7 = base

3 pH of 1 is 100x more acidic than pH of 3 pH of 4 is 1,000x more basic than pH of 1 pH of 6 is 10,000 less concentrated [H + ] than pH of 2 pH[H + ] ions in scientific notation [H + ] ions traditionally 01x x x x x x x x x x x x x x x

4 Properties of Water

5 water will self-ionize to a very small extent in pure water at 25°C, the [concentration] of H + (or H 3 O + ) and OH - ions are equal – [H + ] = [OH - ] – both have a concentration of 1.0 x M the ionic product (the [H2O] is considered a constant so left out) constant (K w ) for water at 298 K is: K w = [H + ] [OH - ] = 1.0 x x 1.0 x = 1.0 x

6 acid solutions have a greater concentration of H + ions (or H 3 O + ) than OH - ions basic solutions are opposite and have more OH -, referred to as alkaline solutions when in water

7 pH = - log [H+] [H+] = 10 -pH Example: If [H + ] = 1 X , what is the pH? –pH = - log 1 X –pH = - (- 10) –pH = 10 Example: If [H + ] = 1.8 X –pH = - log 1.8 X –pH = - (- 4.74) –pH = 4.74 pH calculations – Solving for pH

8 Try These! Find the pH of these: 1)A 0.15 M solution of hydrochloric (HCl) acid –pH =.82 2) A 3.00 X M solution of nitric acid (H 2 NO 3 ) –pH = 6.5

9 pH calculations – Solving for H+ If the pH of Coke is 3.12, what is the [H + ]? 10 -pH = [H + ] [H + ] = = 7.6 x M

10 A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution?A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H + ] = [H + ] 3.16 X = [H + ] pH = - log [H + ] = [H + ] 3.16 X = [H + ]

11 solving for pOH since acids and bases are opposites, pH and pOH are oppositessince acids and bases are opposites, pH and pOH are opposites pOH does not really exist, but it is useful for changing bases to pHpOH does not really exist, but it is useful for changing bases to pH pOH looks at the perspective of a basepOH looks at the perspective of a base pOH = - log [OH - ] Since pH and pOH are on opposite ends, pH + pOH = 14

12 What is the pH of the M NaOH solution? [OH-] = M pOH = - log pOH = 3 pH = 14 – 3 = 11

13 pH [H + ] [OH - ] pOH

14

15 pH testing There are several ways to test pHThere are several ways to test pH –Blue litmus paper (turns red = acid) –Red litmus paper (turns blue = basic) –pH paper (multi-colored) –pH meter (7 is neutral, 7 base) –Universal indicator (multi-colored) –Indicators like phenolphthalein –Natural indicators like red cabbage, radishes

16 pH meter tests the voltage of the electrolytetests the voltage of the electrolyte converts the voltage to pHconverts the voltage to pH must be calibrated with a buffer solution and stored in a solutionmust be calibrated with a buffer solution and stored in a solution

17 pH indicators indicators are dyes that can be added that will change color in the presence of an acid or base. some indicators only work in a specific range of pH once the drops are added, the sample is ruined some dyes are natural, like radish skin or red cabbage