Ch. 8: Acids and Bases

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Ch. 8 Acids and Bases

• Chem 20
• El Camino College

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Two Acid-Base Theories

• Arrhenius Theory
• An acid solution contains more H ions than OH-
  ions
• A base solution contains more OH- ions than H
  ions

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Two Acid-Base Theories

• Note--H is a proton
• Bronsted-Lowry Theory
• An acid is a proton donor
• A base is a proton acceptor

HBr(aq) H2O(l) ? H3O(aq) Br- (aq)
acid
base
NH3(aq) H2O(l) ? NH4(aq) OH- (aq)
acid
base
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Conjugate Acid-Base Pairs

• Conjugate Acid-Base Pairs differ by one H
• The reactants side has the acid and the base
• The products side has the conjugate acid and the
  conjugate base

HBr(aq) H2O(l) D H3O(aq) Br- (aq)
conjugate acid
conjugate base
acid
base
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Conjugate Acid-Base Pairs

• Acid H donor on left side
• Conjugate base missing H on right side
• Base H acceptor on left side
• Conjugate acid received H on right side

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Conjugate Acid-Base Pairs

• Label and link conjugate acid-base pairs.

NH3(aq) CH3CO2H(aq) D NH4(aq) CH3CO2- (aq)
conjugate acid
conjugate base
acid
base
H2SO4(aq) HSO3-(aq) D HSO4-(aq) H2SO3 (aq)
conjugate base
acid
base
conjugate acid
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The Water Equilibrium
H(aq) OH-(aq)

• H2O(l)

Kw (concentration H in M) (conc. OH- in M) Kw
HOH- 1.0 x 10-14
In aq. soln, if HOH- , the soln is
neutral. In aq. soln, if HgtOH- , the soln is
acidic. In aq. soln, if HltOH- , the soln is
basic.
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Examples

• Use this equation HOH- 1.0 x 10-14
• Ex. If the conc. of H is 3.5 x 10-3 M, find OH-
  . Is the solution acidic or basic?

OH- 1 x 10-14 H
1 x 10-14 3.5x10-3M
2.9 x 10-12 M
acidic
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Examples

• Ex. If the conc. of H is 9.9 x 10-11 M, find
  OH- . Is the solution acidic or basic?

OH- 1 x 10-14 H
1 x 10-14 9.9x10-11M
1.0 x 10-4 M
basic
Ex. If OH- is 1.7 x 10-10 M, find H. Is
the solution acidic or basic?
H 1 x 10-14 OH-
1 x 10-14 1.7x10-10M
5.9 x 10-5 M
acidic
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pH Values

• pH 7 is a neutral solution
• pH lt 7 is an acidic solution
• pH gt 7 is a basic solution

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Table 18-2, p. 516
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pH -log H

• Ex. If H 1.5 x 10-6 M, find OH- and pH

OH- 1 x 10-14 H
1 x 10-14 1.5x10-6M
6.7 x 10-9 M
pH -log H -log(1.5 x 10-6) 5.82
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pH -log H

• Ex. If OH- 3.3 x 10-4 M, find H and pH

H 1 x 10-14 OH-
1 x 10-14 3.3x10-4M
3.0 x 10-11 M
pH -log H -log(3.0 x 10-11)
10.52
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pH -log H

• Ex. If H 8.5 x 10-1 M, find OH- and pH

OH- 1 x 10-14 H
1 x 10-14 8.5x10-1M
1.2 x 10-14 M
pH -log H -log(8.5 x 10-1) 0.07
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pH to H

• H 10-pH or antilog(-pH)
• The minus sign goes on the pH value first

Ex. If the pH 5.55, find H
H 10-pH 10-5.55 2.8 x 10-6 M
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pH to H
Ex. If the pH 8.88, find H
H 10-pH 10-8.88 1.3 x 10-9 M
Ex. If the pH 13.00, find H and OH-
H 10-pH 10-13.00 1.0 x 10-13 M
OH- 1 x 10-14 H
1 x 10-14 1.0x10-13M
1.0 x 10-1 M
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Titration

• In the acid-base titration well do in lab, a
  flask contains a mixture of acid and
  phenolphthalein (a ccs)
• Base is added by buret
• When the soln turns pale pink for 30 seconds,
  moles acid moles base.

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Fig. 16-8, p. 457
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p. 459
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p. 459
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p. 459
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Titration How many mL?

• Ex. The flask is filled with 25.00 mL of a 0.500
  M HCl soln. How many mL of 0.300 M NaOH soln will
  neutralize the acid?
• HCl(aq) NaOH(aq) ? NaCl(aq) H2O(l)
• Start with volume of acid, convert to L
• Use molarity of acid as a conversion factor
• Use a mole ratio to convert mol acid to mol base
• Use molarity of base as a conversion factor,
  convert to mL

25.00mL HCl
41.7 mL
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Titration How many mL?

• Ex. The flask is filled with 75.00 mL of a 0.200
  M HCl soln. How many mL of 0.150 M NaOH soln will
  neutralize the acid?

75.00mL HCl
100. mL

• Ex. The flask is filled with 55.00 mL of a 1.5 M
  HCl soln. How many mL of 0.90 M NaOH soln will
  neutralize the acid?

55.00mL HCl
91.7 mL
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Titration Find Molarity

• Ex. The flask is filled with 30.00 mL of a 0.100
  M HCl soln. What is the molarity of the NaOH soln
  if it takes 23.45 mL to neutralize the acid?
• Start with volume of acid, convert to L
• Use molarity of acid as a conversion factor
• Use a mole ratio to convert mol acid to mol base
• Solve for mol base
• Divide mol base by mL base to find molarity.
  Convert to L.

30.00mL HCl
0.00300 mol NaOH
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Titration Find Molarity

• Ex. The flask is filled with 45.00 mL of a 0.996
  M HCl soln. What is the molarity of the NaOH soln
  if it takes 52.33 mL to neutralize the acid?

45.00mL HCl
0.0448 mol NaOH
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Titration Find Molarity

• Ex. The flask is filled with 80.00 mL of a 2.30 M
  H2SO4 soln. What is the molarity of the NaOH soln
  if it takes 70.00 mL to neutralize the acid?
• H2SO4(aq) 2 NaOH(aq) ? Na2SO4 (aq) 2 H2O(l)

80.00mL H2SO4
0.184 mol NaOH
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Strong and Weak Acids

• Strong acids completely break down into ions when
  dissolved in water
• Weak acids only break down into a few ions in
  water. Most of the weak acid molecules stay
  together in water.

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Strong and Weak Acids

• These are the only strong acids
• HCl hydrochloric acid
• HBr hydrobromic acid
• HI hydroiodic acid
• HNO3 nitric acid
• H2SO4 sulfuric acid
• HClO3 chloric acid
• HClO4 perchloric acid.

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Strong and Weak Acids

• All other acids are weak acids. Here are some
  examples
• HF hydrofluoric acid
• H3PO4 phosphoric acid
• CH3CO2H acetic acid
• H2CO3 carbonic acid.

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Buffers

• When a small amount of acid or base is added to
  pure water, pH changes dramatically
• A buffer resists change in pH when small amounts
  of acids or bases are added
• Blood is buffered in the body to a pH of 7.4
• A buffer is a combination of a weak acid and its
  conjugate base (found in an ionic cmpd)
• A strong acid cannot make a buffer.

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How Buffers Work

• This is a buffer made of CH3COOH and CH3COONa
• When acid is added, the extra H reacts with
  CH3COO- to form more CH3COOH
• When base is added, the extra OH- reacts with
  CH3COOH to form more CH3COO-

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Buffers

• Which of the following represents a buffer
  system?
• HCl and NaCl
• HF and NaF
• HNO3 and NaNO3
• CH3COOH and CH3COONa

no
yes
no
yes
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Practice Conjugate Acid-Base Pairs

• Label and link conjugate acid-base pairs.

HNO3(aq) CH3CO2- (aq) D CH3CO2H (aq) NO3- (aq)
HCl(aq) SO42-(aq) D Cl-(aq)
HSO4- (aq)
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Practice pH to H
Ex. If the pH 3.68, find H and OH- in
scientific notation.
H 10-pH 10-3.68 2.1 x 10-4 M
OH- 1 x 10-14 H
1 x 10-14 2.1x10-4M
4.8 x 10-11 M
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Practice Titration

• Ex. The flask is filled with 20.50 mL of a 0.0996
  M HCl soln. How many mL of 0.194 M NaOH soln will
  neutralize the acid?

20.50mL HCl
10.5 mL
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Practice Titration

• Ex. The flask is filled with 25.00 mL of a 2.5 M
  HCl soln. How many mL of 0.50 M Ca(OH)2 soln will
  neutralize the acid?
• 2 HCl(aq) Ca(OH)2 (aq) ? CaCl2 (aq) 2 H2O(l)

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Practice Titration

• Ex. The flask is filled with 20.60 mL of a
  0.09662 M HCl soln. What is the molarity of the
  NaOH soln if it takes 20.92 mL to neutralize the
  acid?

20.60mL HCl
0.001990 mol NaOH