Chem 18 Lecture 3 Exam 3 Common Ion Effect, Buffers and Indicators
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Transcript of Chem 18 Lecture 3 Exam 3 Common Ion Effect, Buffers and Indicators
Common Ion Effect, Buffers and Indicators
Common Ion Effect
• Shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.
• Special case of the Le Châtelier’s principle
Common Ion Effect
• CH3COONa and CH3COOH in solution
CH3COONa(s) → CH3COO-(aq) + Na+
(aq)
CH3COOH(aq) + H2O ↔ CH3COO-(aq) + H3O+
(aq)
Addition of CH3COO- will suppress the ionization of CH3COOH
Henderson-Hasselbalch equation
For a weak acid HA:(derive please)
pH = pKa + log
NOTE: we use [ ]i and not [ ]eq as long as [ ]i≥0.1M
Calculate….
• What is the pH of a 0.30M HCOOH solution? pKa= 3.77
• What will be the pH in the presence of 0.52M HCOOK?
Henderson-Hasselbalch equation
• For a weak base and a salt with a common ion
• pOH = pKb + log
Buffer Solutions
• Solution of (1) a weak acid/base and (2) its salt.
• Both components must be present
• The solution has the ability to resist pH changes upon the addition of either an acid or a base.
Buffer Solutions
• For a simple acetic acid/acetate buffer
CH3COONa(s) → CH3COO-(aq) + Na+
(aq)
CH3COO-(aq) + H+
(aq) → CH3COOH(aq)
CH3COOH(aq) + OH-(aq) → CH3COO-
(aq) + H2O
Buffer Solutions
• See animation
Analyze….
• Which of the following are buffer systems?
• KH2PO4/H3PO4
• NaClO4/HClO4
• C5H5N(weak base)/C5H5NHCl (C5H5N+H is a weak acid)
• KF/HF• KBr/HBr
Buffering Capacity
• Effectiveness of a buffer• Ability to resist pH changes• Depends on the amount of acid and conjugate
base • Amount of acid/base needed to
decrease/increase the pH by 1.0• pH = pKa (maximum buffering capacity)
Buffer range/pH range
• pH range over which a buffer is effective
pH = pKa ± 1
Calculate….
• What is the pH of a buffer system containing 1.0 M CH3COOH and 1.0 M CH3COONa?
• What is the pH after adding 0.10 mole of HCl to 1.0 L of the solution?
• What is the pH after adding 0.10 mole of NaOH to 1.0 L of the solution?
• NOTE: Assume no volume changes
Preparing a buffer solution
• If [acid] ≈ [conj. base], then log ≈ 0• pH ≈ pKa
Calculate….
• How would you prepare a phosphate buffer with a pH of 7.40?
• Ka1= 7.5 x 10-3
• Ka2= 6.2 x 10-8
• Ka3= 4.8 x 10-13
Calculate….
• How will you prepare a carbonate buffer at pH 10.10? You are provided with carbonic acid, sodium hydrogen carbonate and sodium carbonate.
• Ka1 = 4.2 x 10-7
• Ka2 = 4.8 x 10-11
Acid-Base Indicators
• Weak acids/bases that have distinctly different colors in their HIn and In- forms.
• HIn(aq) ↔ H+(aq) + In-
(aq)
• Acidic: HIn color predominates• Basic: In- color predominates
Acid-Base Indicators
• color of HIn predominates
• color of In- predominates
• If [Hin] ≈ [In-], combination of 2 colors
Acid-Base Indicators
¿listo? Teneís un QUIZ
• What is the pH of a solution that is 0.20 M in NH3 and 0.30 M in NH4Cl? Compare its pH in a 0.20 M NH3 solution. Kb = 1.8 x 10-5
• Mr. Eugenio was asked to prepare a buffer at pH = 8.60, using one of the ff. weak acids, HA (Ka = 2.7 x 10-3), HB (Ka = 4.4 x 10-6), HC (Ka = 2.6 x 10-9) Which acid did Mr. Eugenio choose (well, if he performed it correctly hehehe)?