Acid-BaseTitration & pH

16-1 Objectives

1. Describe the self-ionization of water

2. Define pH and give the pH of a neutral solution at 25oC

3. Explain and use the pH scale

4. Given concentrations of H+ & OH-, calculate pH

5. Given pH, calculate concentrations of H+ & OH-

Self-Ionization of Water

Two water molecules produce a hydronium ion and a hydroxide ion by transfer of a proton

Conductivity shows concentrations of H3O+

and are 1.0 x 10-7 mol/L at 25oC

Kw = [H3O+][OH-] = 1.0 x 10-14 M2

(aq)OH aq)(OH O(l)H O(l)H -322

Self-Ionization of Water

Neutral, Acidic, and Basic Solutions

Neutral solutions: have equal [H+] and [OH-]

Acidic Solutions: have greater [H+] than [OH-]

Basic Solutions (alkaline): have greater [OH-] than [H+]

Calculating [H+] and [OH-]

Kw = [H+] [OH-]

If given the concentration of one ion, the concentration of the other can be calculated

Example

A 1.0 x 10-4 M solution of HNO3 has been prepared for a lab experiment. Calculate [H3O

+] and [OH-]

HNO3 is a strong acid so assume 100% dissociation

HNO3(l) + H2O(l) H3O+(aq) + OH-(aq)

Solution

Given: Kw = [H+] [OH-]

1.0 x 10-4 M HNO3

Find concentration of H+:

OH ofmolarity solution L

OH molHNO mol 1

OH mol 1solution L

HNO mol3

3

3

33

Solution

Solve algebraically:

Substitue: ][H

K]OH[

w-

M 10 x 1.0 ]OH[

10 x 1.0

10 x 0.1]OH[

10--

4-

14--

pH Scale

• pH – pouvior hydrogène or “hydrogen power”

• pH is defined as the negative logarithm of the hydronium ion concentration [H3O

+]

• To calculate pH

pH = -log[H3O+]

or

pH = -log[H+]

pH Scale

• To calculate pOH

pOH = -log[OH-]

• At 25oC, [H+] = [OH-] = 1.0 x 10-7 = pH =7

• If [H+] > [OH-] the solution is acidic

• If [OH-] > [H+] the solution is basic

Examples:

1. What is the pH of 1.0 x 10-3 M NaOH solution?

2. What is the pH of a solution if the [H3O+] is

3.4 x 10-5 M?

3. Determine the [H3O+] of an aqueous

solution that has a pH of 4.0.4. The pH of a solution is found to be 7.52.

a) Find [H3O+]

b) Find [OH-] c) Is the solution acidic or basic?

16-2 Objectives

• Describe how a pH indicator works

• Explain an acid-base titration

• Calculate molarity of a solution from titration data

Indicators and pH Meters

Acid-Base Indicator – compounds whose colors are sensitive to pH

In acidic solutions, In- ions act as B-L bases

In basic solutions, OH- react with H+ from indicator

InIn H H

Indicators

Transition Interval – the pH range over which an indicator changes color

Universal Indicators – combination of different indicators

pH meters – determine pH by measuring the voltage difference between two electrodes

Titrations

The controlled addition of and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration

Equivalence Point – the point in a titration where [H+] and [OH-] are present in chemically equivalent amounts

End Point – the point of the titration at which the indicator changes color

Molarity and Titration

Standard Solution – the solution that contains the precisely known concentration of a solute

Primary Standard – a highly purified solid compound used to check the concentration of the known solution in a titration

Calculating Molarity

1. Write balanced chemical equation and determine chemical equivalents

2. Determine moles of acid or base from the known solution used during the titration

3. Determine moles of solute of the unknown used during the titration

4. Determine molarity

Example: in a titration, 27.4 mL of 0.0154 M Ba(OH)2 is added to a 20.0 mL sample of HCl solution of unknown concentration…find the molarity of the acid solution

Solution