# The Chemistry of Acids and BasesPage 6

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Equilibria Involving A Weak Acid

Calculate the pH of a 0.0010 M solution of formic acid, HCO2H.

HCO2H + H2O  HCO2- + H3O+

Ka = 1.8 x 10-4

Approximate solution

[H3O+] = 4.2 x 10-4 M, pH = 3.37

Exact Solution

[H3O+] = [HCO2-] = 3.4 x 10-4 M

[HCO2H] = 0.0010 - 3.4 x 10-4 = 0.0007 M

pH = 3.47

HONORS ONLY!

Slide 54

Equilibria Involving A Weak Base

You have 0.010 M NH3. Calc. the pH.

NH3 + H2O  NH4+ + OH-

Kb = 1.8 x 10-5

Step 1. Define equilibrium concs. in ICE table

[NH3] [NH4+] [OH-]

initial

change

equilib

0.010 0 0

-x +x +x

0.010 - x x x

HONORS ONLY!

Slide 55

Equilibria Involving A Weak Base

You have 0.010 M NH3. Calc. the pH.

NH3 + H2O  NH4+ + OH-

Kb = 1.8 x 10-5

Step 1. Define equilibrium concs. in ICE table

[NH3] [NH4+] [OH-]

initial

change

equilib

0.010 0 0

-x +x +x

0.010 - x x x

HONORS ONLY!

Slide 56

Equilibria Involving A Weak Base

You have 0.010 M NH3. Calc. the pH.

NH3 + H2O  NH4+ + OH-

Kb = 1.8 x 10-5

Step 2. Solve the equilibrium expression

Assume x is small, so

x = [OH-] = [NH4+] = 4.2 x 10-4 M

and [NH3] = 0.010 - 4.2 x 10-4 ≈ 0.010 M

The approximation is valid !

HONORS ONLY!

Slide 57

Equilibria Involving A Weak Base

You have 0.010 M NH3. Calc. the pH.

NH3 + H2O  NH4+ + OH-

Kb = 1.8 x 10-5

Step 3. Calculate pH

[OH-] = 4.2 x 10-4 M

so pOH = - log [OH-] = 3.37

Because pH + pOH = 14,

pH = 10.63

HONORS ONLY!

Slide 58

Types of Acid/Base Reactions: Summary

HONORS ONLY!

Slide 59

pH testing

There are several ways to test pH

Blue litmus paper (red = acid)

Red litmus paper (blue = basic)

pH paper (multi-colored)

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

Universal indicator (multi-colored)

Indicators like phenolphthalein

Natural indicators like red cabbage, radishes

Slide 60

Paper testing

Paper tests like litmus paper and pH paper

Put a stirring rod into the solution and stir.

Take the stirring rod out, and place a drop of the solution from the end of the stirring rod onto a piece of the paper

Read and record the color change. Note what the color indicates.

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