Chapter 16

Acid-Base Titration and pH

Aqueous Solutions and the Concept of pH Self-ionization of water – 2 water

molecules produce a hydronium ion and a hydroxide ion by transferring a protonH20 + H20 H30+ + OH-

Concentration of hydronium ion and the hydroxide ions are represented as [H30+] and [OH-]

For pure water the [H30+] and [OH-] are both 1.0 x 10-7 M

Ion product constant for water (Kw) is obtained by multiplying the [H30+] and [OH-] Kw for room temperature is 1.0 x 10-14 M2, but

varies with temperature

Neutral, acidic, and basic solutions

Neutral solutions have equal [H30+] and [OH-]

Acidic solutions have a greater [H30+] than [OH-]

Basic solutions have a lower [H30+] than [OH-]

Calculating [H30+] and [OH-]

Kw = [H30+] x [OH-] Remember use Kw = 1.0 x 10-14 M2

Ex: A 1.0 x 10-4 M solution of HNO3 has been prepared. What is its [H30+]? What is its [OH-]?

The pH scale

pH – the negative logarithm of the hydronium ion concentrationpH = - log [H30+]

pOH – the negative logarithm of the hydroxide ion concentrationpOH = -log [OH-]

pH + pOH = 14.0

pH less than 7 is acidic pH greater than 7 is basic pH equal to 7 is neutral

Calculations involving pH

Sig figs for pH are different because of the logarithm

There must be as many sig figs to the right of the decimal for the pH as what there were in the [H30+]Ex. [H30+] is 1 x 10-7 has one sig fig and his

pH is 7.0

What is the pH of a solution if the [H30+] is 3.4 x 10-5 M?

pH = - log [3.4 x 10-5 M] pH = 4.47

Finding [H30+] from pH

pH = - log [H30+] can be rearranged to solve for [H30+] by using antilog

[H30+] = 10-pH

Determine the hydronium ion concentration of an aqueous solution that has a pH of 4.0.

pH = -log [H30+]

4.0 = -log [H30+]

10-4.0 = [H30+]

The pH of a solution is determined to be 7.52. What is the pOH, [H30+], and [OH-]?

The molarity of strong acids and bases can be used directly to calculate pH, but not weak acids and weak bases because they don’t ionize/dissociate completely; instead pH must be measured and then [H30+] and [OH-] calculated

Sect. 16-2: Determining pH and Titrations Acid-base indicators – compounds whose

colors are sensitive to pHWeak acids or weak basesDifferent color in the ionized (In-) vs. non-

ionized form (HIn)HIn H+ + In-

In an acid, accepts H+ to form HIn In a base, OH- combines with H+, so more

HIn ionizes to offset loss of H+, thus more In- is present

Transition interval – the pH range over which an indicator changes color If a low pH, then the indicator is a stronger

acid If a higher pH, then the indicator is a weak

acid pH meter – determines the pH of a

solution by measuring the voltage between the two electrodes that are placed in the solution

Titration – the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentrationUsed to determine equivalent volumes of acid

and baseAn example of a chemically equivalent

amount would be 1liter of 0.1M HCl reacting with 0.1 mol solid NaOH

Equivalence point – the point at which the two solutions used in a titration are present in chemically equivalent amounts

End point – the point in a titration at which an indicator changes color

When choosing indicators to be used in a titration remember:Strong acid/strong base will have an

equivalence point at 7Strong acid/weak base will have an

equivalence point below 7Weak acid/strong base will have an

equivalence point above 7

Molarity and titration

Standard solution – the solution that contains the precisely known concentrations of a solute (“known” solution)

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

Steps for determining molarity of unknown solution in titration1. Use balanced neutralization reaction to

determine chemically equivalent amounts of acid and base

2. Determine moles of known solution3. Determine moles of solute of unknown

solution (use stoichiometry)4. Determine molarity of unknown

Example: In a titration, 27.4 mL of 0.0154M Ba(OH)2 is added to a 20.0 mL sample of HCl solution of unknown concentration. What is the molarity of the acid solution?

Ba(OH)2 + 2HCl BaCl2 + 2H2O

1 mol 2 mol 1 mol 2 mol