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Acids and bases, pH and buffers

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Acids and bases, pH and buffers Dr. Mamoun Ahram Lecture 2 Metabolic Alkalosis H+ + HCO3- H2CO3 CO2 + H2O pKa What is pKa? HENDERSON-HASSELBALCH EQUATION The ... – PowerPoint PPT presentation

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Title: Acids and bases, pH and buffers


1
Acids and bases, pH and buffers
  • Dr. Mamoun Ahram
  • Lecture 2

2
Acids and bases
3
Acids versus bases
  • Acid a substance that produces H when dissolved
    in water (e.g., HCl, H2SO4)
  • Base a substance that produces OH- when
    dissolved in water (NaOH, KOH)
  • What about ammonia (NH3)?

4
Brønsted-Lowry acids and bases
  • The Brønsted-Lowry acid any substance able to
    give a hydrogen ion (H-a proton) to another
    molecule
  • Monoprotic acid HCl, HNO3, CH3COOH
  • Diprotic acid H2SO4
  • Triprotic acid H3PO3
  • Brønsted-Lowry base any substance that accepts a
    proton (H) from an acid
  • NaOH, NH3, KOH

5
Acid-base reactions
  • A proton is transferred from one substance (acid)
    to another molecule
  • Ammonia (NH3) acid (HA) ? ammonium ion (NH4)
    A-
  • Ammonia is base
  • HA is acid
  • Ammonium ion (NH4) is conjuagte acid
  • A- is conjugate base

6
Water acid or base?
  • Both
  • Products hydronium ion (H3O) and hydroxide

7
Amphoteric substances
  • Example water
  • NH3 (g) H2O(l) ? NH4(aq) OH(aq)
  • HCl(g) H2O(l) ? H3O(aq) Cl-(aq)

8
Acid-base reactions
  • Acid base ? salt H2O
  • Exceptions
  • Carbonic acid (H2CO3)-Bicarbobate ion (HCO3-)
  • Ammonia (NH3)-

9
Acid/base strength
10
Rule
  • The stronger the acid, the weaker the conjugate
    base
  • HCl(aq) ? H(aq) Cl-(aq)
  • NaOH(aq) ? Na(aq) OH-(aq)
  • HC2H3O2 (aq) ? H(aq) C2H3O2-(aq)
  • NH3 (aq) H2O(l) ? NH4(aq) OH-(aq)

11
Equilibrium constant
  •    HA  lt--gt   H A-
  • Ka gt1 vs. lt1

12
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13
Expression
  • Molarity (M)
  • Normality (N)
  • Equivalence (N)

14
Molarity of solutions
  • moles grams / MW
  • M moles / volume (L)
  •  
  • grams M x vol (L) x MW

15
Exercise
  • How many grams do you need to make 5M NaCl
    solution in 100 ml (MW 58.4)?
  • grams 58.4 x 5 moles x 0.1 liter 29.29 g

16
Normal solutions
  • N n x M (where n is an integer)
  • n the number of donated H
  • Remember!
  • The normality of a solution is NEVER less than
    the molarity

17
Equivalents
  • The amount of molar mass (g) of hydrogen ions
    that an acid will donate
  • or a base will accept
  • 1M HCl 1M H 1 equivalent
  • 1M H2SO4 2M H 2 equivalents

18
Exercise
  • What is the normality of H2SO3 solution made by
    dissolving 6.5 g into 200 mL? (MW 98)?

19
Example
  • One equivalent of Na 23.1 g
  • One equivalent of Cl- - 35.5 g
  • One equivalent of Mg2 (24.3)/2 12.15 g
  • Howework
  • Calculate milligrams of Ca2 in blood if total
    concentration of Ca2 is 5 mEq/L.

20
Titration
  • The concentration of acids and bases can be
    determined by titration

21
Excercise
  • A 25 ml solution of 0.5 M NaOH is titrated until
    neutralized into a 50 ml sample of HCl. What was
    the concentration of the HCl?
  • Step 1 - Determine OH-
  • Step 2 - Determine the number of moles of OH-
  • Step 3 - Determine the number of moles of H
  • Step 4 - Determine concentration of HCl

22
A 25 ml solution of 0.5 M NaOH is titrated until
neutralized into a 50 ml sample of HCl
  • Moles of base Molarity x Volume
  • Moles base moles of acid
  • Molarity of acid moles/volume

23
Another method
  • MacidVacid MbaseVbase

24
Note
  • What if one mole of acid produces two moles of H
  • MacidVacid 2MbaseVbase

25
Homework
  • If 19.1 mL of 0.118 M HCl is required to
    neutralize 25.00 mL of a sodium hydroxide
    solution, what is the molarity of the sodium
    hydroxide?
  • If 12.0 mL of 1.34 M NaOH is required to
    neutralize 25.00 mL of a sulfuric acid, H2SO4,
    solution, what is the molarity of the sulfuric
    acid?

26
Equivalence point
27
Ionization of water
  • H3O H

28
Equilibrium constant
  • Keq 1.8 x 10-16 M

29
Kw
  • Kw is called the ion product for water

30
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31
pH
32
What is pH?
33
Acid dissociation constant
  • Strong acid
  • Strong bases
  • Weak acid
  • Weak bases

34
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35
pKa
36
What is pKa?
37
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38
Henderson-Hasselbalch equation
39
The equation
pKa is the pH where 50 of acid is dissociated
into conjugate base
40
Buffers
41
Maintenance of equilibrium
42
What is buffer?
43
Titration
44
Midpoint
45
Buffering capacity
46
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47
Conjugate bases
Acid Conjugate base
CH3COOH CH3COONa (NaCH3COO)
H3PO4 NaH2PO4
H2PO4- (or NaH2PO4) Na2HPO4
H2CO3 NaHCO3
48
How do we choose a buffer?
49
Problems and solutions
  • A solution of 0.1 M acetic acid and 0.2 M acetate
    ion. The pKa of acetic acid is 4.8. Hence, the pH
    of the solution is given by
  • Similarly, the pKa of an acid can be calculated

50
Exercise
  • What is the pH of a buffer containing 0.1M HF and
    0.1M NaF? (Ka 3.5 x 10-4)

51
Homework
  • What is the pH of a solution containing 0.1M HF
    and 0.1M NaF, when 0.02M NaOH is added to the
    solution?

52
At the end point of the buffering capacity of a
buffer, it is the moles of H and OH- that are
equal
53
Exercise
  • What is the concentration of 5 ml of acetic acid
    knowing that 44.5 ml of 0.1 N of NaOH are needed
    to reach the end of the titration of acetic acid?
    Also, calculate the normality of acetic acid.

54
Polyprotic weak acids
  • Example

55
Hence
56
Excercise
  • What is the pH of a lactate buffer that contain
    75 lactic acid and 25 lactate? (pKa 3.86)
  • What is the pKa of a dihydrogen phosphae buffer
    when pH of 7.2 is obtained when 100 ml of 0.1 M
    NaH2PO3 is mixed with 100 ml of 0.1 M Na2HPO3?

57
Buffers in human body
  • Carbonic acid-bicarbonate system (blood)
  • Dihydrogen phosphate-monohydrogen phosphate
    system (intracellular)
  • Proteins

58
Blood buffering
Blood (instantaneously)
CO2 H20
H2CO3
H HCO3-
Lungs (within minutes)
Excretion via kidneys (hours to days)
59
Roles of lungs and kidneys
  • Maintaining blood is balanced by the kidneys and
    the lungs
  • Kidneys control blood HCO3 concentration (HCO3)
  • Lungs control the blood CO2 concentration (PCO2)

60
Calculations
61
Acidosis and alkalosis
  • Can be either metabolic or respiratory
  • Acidosis
  • Metabolic production of ketone bodies
    (starvation)
  • Respiratory pulmonary (asthma emphysema)
  • Alkalosis
  • Metabolic administration of salts or acids
  • Respiratory hyperventilation (anxiety)

62
Acid-Base Imbalances
  • pHlt 7.35 acidosis
  • pH gt 7.45 alkalosis

63
Respiratory Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
64
Respiratory Alkalosis
H HCO3- ? H2CO3 ? CO2 H2O
65
Metabolic Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
66
Metabolic Alkalosis
H HCO3- ? H2CO3 ? CO2 H2O
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