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Ch. 18- Acids & Bases

II. pH

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A. Ionization of Water

H2O + H2O H3O+ + OH-

Kw = [H3O+][OH-] = 1.0 10-14

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A. Ionization of Water

Find the hydroxide ion concentration of 3.0 10-2 M HCl.

[H3O+][OH-] = 1.0 10-14

[3.0 10-2][OH-] = 1.0 10-14

[OH-] = 3.3 10-13 M

Acidic or basic?Acidic

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pH = -log[H3O+]

B. pH Scale

0

7INCREASING

ACIDITY NEUTRALINCREASING

BASICITY

14

pouvoir hydrogène (Fr.)“hydrogen power”

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B. pH Scale

pH of Common SubstancespH of Common Substances

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B. pH Scale

pH = -log[H3O+]

pOH = -log[OH-]

pH + pOH = 14

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B. pH Scale

What is the pH of 0.050 M HNO3?

pH = -log[H3O+]

pH = -log[0.050]

pH = 1.3

Acidic or basic?Acidic

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B. pH Scale

What is the molarity of HBr in a solution that has a pOH of 9.6?

pH + pOH = 14

pH + 9.6 = 14

pH = 4.4

Acidic

pH = -log[H3O+]

4.4 = -log[H3O+]

-4.4 = log[H3O+]

[H3O+] = 4.0 10-5 M HBr

C. Acid-Base Indicators Why measure pH?

Everyday solutions we use - everything from swimming pools, soil conditions for plants, medical diagnosis, soaps and shampoos, etc.

Sometimes we can use indicators, other times we might need a pH meter

Acids Affect Indicators, by changing their color

Blue litmus paper turns red in contact with an acid (and red paper stays red).

Bases Affect Indicators

Red litmus paper turns blue in contact with a base (and blue paper stays blue).

Phenolphthalein turns purple in a base.

How to measure pH with wide-range paper

1. Moisten the pH indicator paper strip with a few drops of solution, by using a stirring rod.

2.Compare the color to the chart on the vial – then read the pH value.

Some of the many pH

Indicators and theirpH range

C. Acid-Base IndicatorsAlthough useful, there are limitations

to indicators:• usually given for a certain

temperature (25 oC), thus may change at different temperatures

• what if the solution already has a color, like paint?

• the ability of the human eye to distinguish colors is limited

C. Acid-Base IndicatorsA pH meter may give more definitive

results• some are large, others portable• works by measuring the voltage

between two electrodes; typically accurate to within 0.01 pH unit of the true pH

• Instruments need to be calibrated

Worksheet and bookwork

Complete worksheet 18.3Bookwork p. 651 problem 22p. 653 24, 25p. 654 27

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