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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