Acids and bases, pH and buffers

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Acids and bases, pH and buffers

• Dr. Mamoun Ahram
• Lecture 2

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Acids and bases
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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)?

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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

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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

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Water acid or base?

• Both
• Products hydronium ion (H3O) and hydroxide

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Amphoteric substances

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

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Acid-base reactions

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

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Acid/base strength
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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)

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Equilibrium constant

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

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Expression

• Molarity (M)
• Normality (N)
• Equivalence (N)

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Molarity of solutions

• moles grams / MW
• M moles / volume (L)
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• grams M x vol (L) x MW

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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

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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

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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

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Exercise

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

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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.

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Titration

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

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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

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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

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Another method

• MacidVacid MbaseVbase

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Note

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

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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?

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Equivalence point
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Ionization of water

• H3O H

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Equilibrium constant

• Keq 1.8 x 10-16 M

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Kw

• Kw is called the ion product for water

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pH
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What is pH?
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Acid dissociation constant

• Strong acid
• Strong bases
• Weak acid
• Weak bases

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pKa
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What is pKa?
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Henderson-Hasselbalch equation
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The equation
pKa is the pH where 50 of acid is dissociated
into conjugate base
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Buffers
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Maintenance of equilibrium
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What is buffer?
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Titration
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Midpoint
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Buffering capacity
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Conjugate bases
Acid Conjugate base
CH3COOH CH3COONa (NaCH3COO)
H3PO4 NaH2PO4
H2PO4- (or NaH2PO4) Na2HPO4
H2CO3 NaHCO3
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How do we choose a buffer?
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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

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Exercise

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

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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?

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At the end point of the buffering capacity of a
buffer, it is the moles of H and OH- that are
equal
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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.

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Polyprotic weak acids

• Example

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Hence
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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?

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Buffers in human body

• Carbonic acid-bicarbonate system (blood)
• Dihydrogen phosphate-monohydrogen phosphate
  system (intracellular)
• Proteins

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Blood buffering
Blood (instantaneously)
CO2 H20
H2CO3
H HCO3-
Lungs (within minutes)
Excretion via kidneys (hours to days)
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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)

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Calculations
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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)

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Acid-Base Imbalances

• pHlt 7.35 acidosis
• pH gt 7.45 alkalosis

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Respiratory Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
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Respiratory Alkalosis
H HCO3- ? H2CO3 ? CO2 H2O
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Metabolic Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
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Metabolic Alkalosis
H HCO3- ? H2CO3 ? CO2 H2O