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Buffer Problems

Buffer Probelms

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Buffer Problems

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  1. Aqueous Equilibrium

  2. Common Ion Effect

  3. Learning Outcomes and Prereqs • Pre-reqs: • Identify how changing concentrations of reactants or products shift an equilibrium (Le Chatelier’s Principle) • Learning Outcomes • Identify the effect of adding a conjugate base of an acid on pH and ionization. • Identify the effect of adding a conjugate acid of a base on pH and ionization.

  4. ReminderLe Chatelier’s Principle • If you apply stress to a system it shifts to relieve the stress. • Reaction shifts left or right to relieve the stress. • Analogy-

  5. The common ion effect • If you have a solution of an acid and its conjugate base the ionization is lessened by the presence of the ion. • Same is true for a base and its conjugate acid • Conjugate acid/base added through it’s salt. • So….what does that mean? Examples?

  6. The common ion effect • Adding a conjugate lessons the effect of ionization. • Examples? • Formic acid and sodium formate • Ammonia and Ammonium Chloride H

  7. Why do we care? If ionization is decreased how does this affect the OH- and H+ concentrations? H Question: If HCOO- is added, what happens to the concentration of H+? What does that do to pH? It decreases [H+]: pH: It increases

  8. But does it really matter? An example to illustrate: Determine the pH of a 0.20M NH3 solution. Then determine the pH of a solution that is 0.20M NH3 and also 0.30M NH4Cl

  9. Great so we can do the calculation problems just like every other problem we’ve done! It’s just a bit more algebra! Or maybe there is an easier way

  10. Review • If you add the conjugate base of an acid: • the ionization will be lessened • This means less [H+] so a higher pH • If you add the conjugate acid of a base: • The ionization will be lessened • This means less [OH-] so a lower pH.

  11. Henderson Hasselbalch Equation

  12. Learning outcomes • Derive the Henderson Hasselbalch Equation • Use the Henderson Hasselbalch equation to solve a problem. • Identify when we are allowed to use this, and when we have to use the previous methods of solving acid/base problems.

  13. Incredibly related quote:

  14. Lets do something similar to the previous problem we did: but now for the most general of cases. If we do it once in general we can use the results.

  15. Henderson-Hasslebalch: Creating the generic equation

  16. Henderson-Hasslebalch: Problem solving tips Use this for buffer problems only. Be sure you are using the Ka NOT the Kb As with all equations, get good at manipulating them. Only use if weak acid approximation is valid

  17. Example: Using the Henderson-Hasslebalch equation • 0.20M CH3COOH and 0.30M CH3COONa are mixed. Find the pH.

  18. Review • The Henderson Hasselbalch equation comes from solving a generic acid problem where both an acid and its conjugate base are present. • You can use this equation to solve problems whenever the 5% approximation is valid (note: you don’t have to show me, but you do need to check in case it doesn’t work)

  19. Introduction to Buffers Motivation and

  20. Learning Outcomes • Identify why our body needs buffers, and what happens if something causes that system to fail. • Define buffers. • Identify how buffers work to keep pH changes small • Identify if two components form a buffer

  21. Real life importance check: pH of blood=7.4 Alkalosis> 7.50 Acidosis <7.35 Some products of your metabolism: Ammonia Urea (a base) Uric acid CO2 (acidic when reacting with water) Urea

  22. Alkalosis and Acidosis Metabolic

  23. Hypoventilation increased CO2 acidosis Alkalosis and Acidosis Respiratory Hyperventilation decreased CO2 alkalosis

  24. Real life importance check: pH of blood=7.4 Alkalosis> 7.50 Acidosis <7.35 Urea Products of your metabolism: Ammonia Urea (a base) Uric acid CO2 (acidic when reacting with water) So how can your body control your pH so well?

  25. Buffers • Acid and its conjugate base, or a base and its conjugate acid • Essentially the systems we’ve talked about in the common ion effect questions • It works by converting a strong acid into a weak acid, or a strong base into a weak base. • A strong base can’t exist in solution with a weak acid • A strong acid can’t exist in solution with a weak base

  26. Activity Part 1: (I’ll need some volunteers)

  27. Buffers • Acid and its conjugate base, or a base and its conjugate acid • Basically the systems we’ve talked about in the common ion effect questions • It works by converting a strong acid into a weak acid, or a strong base into a weak base. • A strong base can’t exist in solution with a weak acid • A strong acid can’t exist in solution with a weak base

  28. Activity Part 2: (I’ll need some volunteers again) Acids react with bases Bases react with acids

  29. Real life importance check: pH of blood=7.4 Alkalosis> 4.5 Acidosis <7.35 Products of your metabolism: Ammonia Urea (a base) Uric acid CO2 (acidic when reacting with water) Urea So how can your body control your pH so well? It uses buffers!!!!

  30. Buffers in the Body: • We discussed how CO2 can become acid in the body • This equilibrium is as follows, if both are present it’s a buffer

  31. Buffers in the Body: Hypoventilation increased CO2 acidosis • How do we regulate CO2 levels? • Breathing levels • What happens to the CO2 if we breath shallowly? • Respiratory acidosis • Why is this bad? • Many reasons: here is one: • Hemoglobin in red blood cells binds either O2 or H+ Hyperventilation decreased CO2 alkalosis

  32. Which of the following are buffers: • yes • no • no • Yes • Yes • Yes • Yes • KNO2/HNO2 • KCl/HCl • KHSO4/ H2SO4 • Na2HPO4/NaH2PO4 • KCN/HCN • Na2SO4/NaHSO4 • NH3/NH4NO3

  33. Review • Buffers prevent large changes in pH caused by the addition of acids or bases. • This is needed in many systems. Our blood buffer system uses buffers to keep the pH in a very narrow range. • Buffers work by having both an acid and its conjugate base, or a base and its conjugate acid present. • Therefore if an acid is added, it reacts with the base • If a base is added, it reacts with the acid • This keeps the pH change small.

  34. Buffer Calculations.

  35. Learning outcomes • Use the Henderson-Hasslebalch equation to solve for the pH of buffers. • Use the Henderson-Hasslebalch equation to solve for the pH of buffers after adding an acid or base. • Test your knowledge of buffers by answering a couple of conceptual questions.

  36. Buffers • Calculate the pH of the solution that results from mixing 0.200L of 0.050M (CH3)2NH with 0.320L of 0.040M (CH3)2NH2Cl. (Kb=5.88x10-4) • You try: • Calculate the pH of the solution that results from mixing 72.0mL of 0.015M (CH3)2NH with 20mL of 0.200M (CH3)2NH2Cl • The pH of blood plasma is 7.40, Assuming the principal buffer system is HCO3- and H2CO3 find the ratio of HCO3- and H2CO3. (Ka=4.2x10-7)

  37. Adding acids and bases Acids react with bases Bases react with acids

  38. Buffers when adding a base or acid • It works by converting a strong acid into a weak acid, or a strong base into a weak base. • A strong base can’t exist in solution with a weak acid • A strong acid can’t exist in solution with a weak base Notice NOT equilibrium arrow!!! Use “new” concentration of acid or base to do equilibrium calculations in the same manner as before.

  39. Steps for buffer problem with addition of acid or base. • Step 1: Decide if it is a buffer • Step 2: Write reaction of buffer with strong acid or base • Step 3: (if needed) do ICF chart (my take on ICE, its not an equilibrium its “final”) to find amount of weak acid or base • Suggested, do this in mols • Step 4: fill into the HH equation if assumptions are valid (Ka/Kb is low, concentrations high) or use ICE chart and Ka or Kbect…as we did with other equilibrium problems. • If using HH equation use moles • If using Ka or Kb equation use molarity

  40. Buffers when adding a base or acid Examples: • Calculate the pH of 1.00L of the buffer 1.00M CH3COONa and 1.00M CH3COOH. Ka=4.2x10-7 • A) before • and after the addition of * • B) 0.080 molNaOHor • C) 0.120 molHCl *assume no change in volume ***Note: You’ll notice your book always uses an ICE chart, filling into K, solving for X, finding pH protocol, I’m not sure why they use this after having taught you the easier way to do it. Choose whichever you like most.

  41. Example Questions: • We said the normal ratio of HCO3-:H2CO3 is 20:1, Is your blood better at buffering added acid or added base?

  42. Review • For most cases you can use the Henderson-Hasslebalch equation for solving buffer problems. • (When the 5% rule is valid, you need to test this for buffers, but you don’t need to show me, otherwise use an ice chart). • If you add an acid or a base to a buffer, first react the species away ( usually using moles) then solve a HH equation.

  43. Chemistry and life segment The biology here will not be tested (however you better know the difference between an alcohol and a carboxylic acid since we talked about that earlier in the class).

  44. Little side note while we are on biological pH topics: Hangovers: Dispelling another myth “Acetic acid causes acidosis and this is why you get a hangover”

  45. Hangovers:   • Metabolic pathway- • Acetaldehyde can cause migraine strength headaches and vomiting. • Acetic acid is harmless • Alcohol decreases the Anti-diuretic hormone (adh) • Increases urine output, Increases dehydration • Depletes electrolytes leading to metabolic acidosis!

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