11

The pH ScaleThe Power or Potential of

Hydrogen

22

pH scale

• the pH scale is a way of the pH scale is a way of expressing the strength of expressing the strength of acids and basesacids and bases

• instead of using very small instead of using very small numbers, we just use the numbers, we just use the NEGATIVE power of 10 on the NEGATIVE power of 10 on the Molarity of the HMolarity of the H++ ion ion

• under 7 = acidunder 7 = acid 7 = neutral 7 = neutral

over 7 = baseover 7 = base

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• 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

0 1x100 1.0

1 1x10-1 0.1

2 1x10-2 0.01

3 1x10-3 0.001

4 1x10-4 0.0001

5 1x10-5 0.00001

6 1x10-6 0.000001

7 1x10-7 0.0000001

8 1x10-8 0.00000001

9 1x10-9 0.000000001

10 1x10-10 0.0000000001

11 1x10-11 0.00000000001

12 1x10-12 0.000000000001

13 1x10-13 0.0000000000001

14 1x10-14 0.00000000000001

44

Properties of Water

55

• water will self-ionize to a very small extent

• in pure water at 25°C, the [concentration] of H+ (or H3O+) and OH- ions are equal– [H+] = [OH-]– both have a concentration of 1.0 x 10-7 M

• the ionic product (the [H2O] is considered a constant so left out) constant (Kw) for water at 298 K is:

Kw = [H+] [OH-] = 1.0 x 10-7 x 1.0 x 10-7

= 1.0 x 10-14

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•acid solutions have a greater concentration of H+ ions (or H3O+) than OH- ions

•basic solutions are opposite and have more OH-, referred to as alkaline solutions when in water

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pH = - log [H+]

[H+] = 10-pH

•Example: If [H+] = 1 X 10-10, what is the pH?– pH = - log 1 X 10-10

– pH = - (- 10)

– pH = 10

•Example: If [H+] = 1.8 X 10-5

– pH = - log 1.8 X 10-5

– pH = - (- 4.74)

– pH = 4.74

pH calculations – Solving pH calculations – Solving for pHfor pHpH calculations – Solving pH calculations – Solving for pHfor pH

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Try These!Try These!

Find the pH of these:Find the pH of these:

1)1) A 0.15 M solution of hydrochloric A 0.15 M solution of hydrochloric (HCl) acid(HCl) acid– pH = .82pH = .82

2) A 3.00 X 102) A 3.00 X 10-7-7 M solution of nitric acid M solution of nitric acid (H(H22NONO33))– pH = 6.5pH = 6.5

99pH calculations – Solving for pH calculations – Solving for H+H+pH calculations – Solving for pH calculations – Solving for H+H+

• If the pH of Coke is 3.12, what is the [H+]?

• 10-pH = [H+]

• [H+] = 10-3.12

• = 7.6 x 10-4 M

1010

• A solution has a pH of 8.5. What is the A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the Molarity of hydrogen ions in the solution?solution?

pH = - log [HpH = - log [H++]]

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

pH = - log [HpH = - log [H++]]

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

1111

solving for pOH

• since acids and bases are opposites, pH since acids and bases are opposites, pH and pOH are oppositesand pOH are opposites

• pOH does not really exist, but it is useful pOH does not really exist, but it is useful for changing bases to pHfor changing bases to pH

• pOH looks at the perspective of a basepOH looks at the perspective of a basepOH = - log [OHpOH = - log [OH--]]

Since pH and pOH are on opposite ends,Since pH and pOH are on opposite ends,

pH + pOH = 14pH + pOH = 14

1212

What is the pH of the What is the pH of the 0.0010 M Na0.0010 M NaOHOH solution? solution?

[OH-] = 0.0010 M[OH-] = 0.0010 M

pOH = - log 0.0010pOH = - log 0.0010

pOH = 3pOH = 3

pH = 14 – 3 = pH = 14 – 3 = 1111

1313

pHpH [H+][H+] [OH-][OH-] pOHpOH

1414

1515pH testing

• There are several ways to test pHThere are several ways to test pH

–Blue litmus paper (turns red = acid)Blue litmus paper (turns red = acid)

–Red litmus paper (turns blue = basic)Red litmus paper (turns blue = basic)

–pH paper (multi-colored)pH paper (multi-colored)

–pH meter (7 is neutral, <7 acid, >7 pH meter (7 is neutral, <7 acid, >7 base)base)

–Universal indicator (multi-colored)Universal indicator (multi-colored)

– Indicators like phenolphthaleinIndicators like phenolphthalein

–Natural indicators like red cabbage, Natural indicators like red cabbage, radishesradishes

1616

pH meter

• tests the voltage of the tests the voltage of the electrolyteelectrolyte

• converts the voltage to converts the voltage to pHpH

• must be calibrated with must be calibrated with a buffer solution and a buffer solution and stored in a solutionstored in a solution

1717pH 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