Acids and Bases(pH and Titrations)

PreAP Chemistry

Chap. 14

The pH Scale

• Remember that as water self-ionizes, it produces equal amounts of H3O+ ions and OH- ions in solution.

• Pure water contains 0.0000001 moles of H3O+

ions in one liter of water, or 1 x 10-7 mol (and an equal number of OH- ions).

• More acidic solutions contain more H3O+ ions and less OH-. Lemon juice, which is a dilute solution of citric acid may contain 1 x 10-3 moles of H3O+

and only 1 x 10-11 moles of OH-

pH

pH = -log [H1+]

Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285

The pH Scale

• The pH scale was developed as a way of comparing the relative concentrations of H3O+

and OH- ions in solution.

• The basic equation is pH = -log[H3O+]

So, if pure water has a hydronium ion concentration of 1 x 10-7, it has a pH of 7

Lemon juice, with a 1 x 10-3 concentration would have a pH of 3

pH of Common Substance

14 1 x 10-14 1 x 10-0 013 1 x 10-13 1 x 10-1 112 1 x 10-12 1 x 10-2 211 1 x 10-11 1 x 10-3 310 1 x 10-10 1 x 10-4 49 1 x 10-9 1 x 10-5 58 1 x 10-8 1 x 10-6 6

6 1 x 10-6 1 x 10-8 85 1 x 10-5 1 x 10-9 94 1 x 10-4 1 x 10-10 103 1 x 10-3 1 x 10-11 112 1 x 10-2 1 x 10-12 121 1 x 10-1 1 x 10-13 130 1 x 100 1 x 10-14 14

NaOH, 0.1 MHousehold bleachHousehold ammonia

Lime waterMilk of magnesia

Borax

Baking sodaEgg white, seawaterHuman blood, tearsMilkSalivaRain

Black coffeeBananaTomatoesWineCola, vinegarLemon juice

Gastric juice

Mo

re b

asic

Mo

re a

cid

icpH [H1+] [OH1-] pOH

7 1 x 10-7 1 x 10-7 7

Acid – Base Concentrations

pH = 3

pH = 7

pH = 11

OH-

H3O+OH-

OH-H3O+

H3O+

[H3O+] = [OH-][H3O+] > [OH-] [H3O+] < [OH-]

acidicsolution

neutralsolution

basicsolution

con

cen

trat

ion

(m

ole

s/L)

10-14

10-7

10-1

Timberlake, Chemistry 7th Edition, page 332

pH of Common Substances

Timberlake, Chemistry 7th Edition, page 335

© 2009, Prentice-Hall, Inc.

Titration

In this technique a known concentration of base (or acid) is slowly added to a solution of acid (or base).

© 2009, Prentice-Hall, Inc.

Titration

A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.

© 2009, Prentice-Hall, Inc.

Titration of a Strong Acid with a Strong Base

From the start of the titration to near the equivalence point, the pH goes up slowly.

© 2009, Prentice-Hall, Inc.

Titration of a Strong Acid with a Strong Base

Just before (and after) the equivalence point, the pH increases rapidly.

© 2009, Prentice-Hall, Inc.

Titration of a Strong Acid with a Strong Base

At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

© 2009, Prentice-Hall, Inc.

Titration of a Strong Acid with a Strong Base

As more base is added, the increase in pH again levels off.