pH and Titrations

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pH and Titrations

• You will need calculators for this part.

2
The degree of conductivity of pure water is very
small, but pure water does conduct electricity.

• What is the source of ions in pure water?
• H2O H2O(l) ? H3O(aq) OH-(aq)
• Concentration of H3O OH- constant

3
Is pure water acidic or basic?

• Neutral equal numbers of H3Oand OH-
• (Concentration of H3Oand OH- must be the same
  because one of each ion is formed when two water
  molecules react. Since there is never more of
  any one type of ion, pure water is neutral.)

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H and OH- Concentration

• Observe relationship between hydronium and
  hydroxide ions (both present in both acidic and
  basic solutions)

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When H (H3O) increases, OH- decreases, and vice
versa.

• Substance H3O OH-
• Tomatoes 10-4 10-10
• Potatoes 10-6 10-8
• Milk of magnesia 10-10 10-4
• The stronger the acid, the greater the relative
  number of hydronium ions the reverse is true for
  bases.

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H3O Concentration pH

• 1 x 10-1 M 1
• 1 x 10-3 M 3
• 1 x 10-5 M 5
• 1 x 10-7 M 7
• 1 x 10-9 M 9
• 1 x 10-11 M 11
• 1 x 10-13 M 13

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H3O Concentration pH

• 1 x 10-1 M 1
• 1 x 10-3 M 3
• 1 x 10-5 M 5
• 1 x 10-7 M 7
• 1 x 10-9 M 9
• 1 x 10-11 M 11
• 1 x 10-13 M 13

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Summary pH Scale

• Stands for power of hydrogen
• Based on equilibrium concentration of hydronium
  ions in water
• pH - log H3O
• Molarity
• Ex H3O 1 x 10-3 M
• - log 1 x 10-3 pH
• pH 3

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pOH -log OH-

• At neutrality, H3O OH-
• H3O OH- 1 x 10-7
• pH 7
• pOH7
• pH pOH 14

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Make this chart
1 x 10-3
11
3
1 x 10-11
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Example

• At 298K, the H ion concentration of an aqueous
  soln is 1.0 x 10-5 M. What is the OH-
  concentration? Is the solution acid, base or
  neutral?
• pH -log 1.0 x 10-5
• pH 5
• pOH 14-59
• OH- 1.0x10-9M
• Solution is acidic.

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Examples

• 1. What is the pH of a soln with H of 1.0 x
  10-8? Is the soln acidic, basic or neutral?
• pH -logH -log(1.0x10-8)
• pH 8 basic

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• 2. What is the pH of a solution with H
  1.0x10-5? Is it an acid, base or neutral?
• pH -logH -log(1.0x10-5)
• pH 5 acid

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• 3. What is the pH of a solution whose OH- is
  1.0x10-10?
• pOH -logOH- -log(1x10-10)
• pOH 10
• pH 14-10 4 acidic

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pH and pOH

• pH is the negative log of the hydrogen ion (or
  hydronium ion) concentration.
• pH -logH
• pH lt7 acid
• pHgt7 base (or alkaline)
• pH 7 is neutral
• pOH -logOH-
• pH pOH 14

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Do practice prob. p.484 (1-4)

• Do WS.

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Calculating ion concentration from pH

• H antilog(-pH)
• Example What are the H and OH- in a healthy
  persons blood that has a pH of 7.40?
• H antilog (-pH) antilog (7.40)
• H 4.0 x 10-8 M
• pOH 14 7.40 6.60

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• OH- antilog (-pOH) antilog(-6.60)
• OH- 2.5 x 10-7

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Titrations

• Titration process to determine equivalent
  amounts of reactants in a soln
• Equivalence point Point in titration where
  moles of H from the acid equals moles of OH-
  from the base.
• Endpoint point where indicator used in
  titration changes color.

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Titration Curve
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Titration Calculations

• Ex. 1 When 42.5mL of 1.03M NaOH is added to
  50.0mL of vinegar, the phenolphthalein in the
  solution just turns pink. What is the
  concentration of acetic acid in the vinegar?
• V1M1 V2M2
• (0.0425L)(1.03M) (0.0500L)M2
• M2 0.876M

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Ex. 2

• What is the concentration of NH3in household
  ammonia if 48.25mL of 0.5248M HCl is needed to
  neutralize 22.00mL of the ammonia?
• V1M1 V2M2
• (0.04825L)(0.5248M) (0.02200L)M2
• M2 1.151M

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Ex. 3

• What is the concentration of 50.0mL of a NaOH
  soln that is neutralized by 30.0mL of 0.500MHCl?
• V1M1 V2M2
• (0.0300L)(0.500M) (0.0500L)M2
• M2 0.300M NaOH

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Indicators

• Indicator substance that is one color in an
  acidic soln and another in a basic soln. have pH
  ranges over which color changes
• Acids turn Blue Litmus Red
• Phenolphthalein colorless in an acid
• Bases turn Red litmus Blue
• Phenolphthalein pink in a base

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IndicatorsSee p.495!

• Bromthymol blue
• Yellow below 6.0
• Green 6.0-7.6
• Blue above 7.6
• Phenolphthalein
• Colorless below 8.0
• Faint pink 8.0-10.0
• Bright fuschia above 10.0
• Neutral Litmus Paper

• Universal Indicator
• Red-very strongly acidic 2.0
• Orange-strongly acidic 4.0
• Yellow-weakly acidic 6.0
• Green-weakly basic 8.0
• Blue strongly basic 10.0
• Purple-very strongly basic 12.0

Click above pic or see next slide for Cabbage
juice indicators.
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Determining pH of Various Sub. Pre-lab Directions

• Pipets tip up! Dont contaminate!
• 1 drop only!
• Describe color AND approximate pH
• Heading, title, purpose, data table in journal
  (keep clean paper copy for final draft).

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Do Practice Prob. p614 (2122)

• 21. a) H 4.3x10-3M
• OH- 2.3 x 10-12M
• b) H 8.9 x 10-12M
• OH- 1.1 x 10-3M
• c) H 3.2 x 10-7M
• OH- 3.2 x 10-8M

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• 22. a) pH 0.00
• b) pH 1.30
• c) pH 14.00
• d) pH 9.68

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Neutralization

• Neutralization reaction between an acid and a
  base products are water and an ionic compound
  (called a salt)
• KOH HCl ? KCl H2O
• Complete Neutralization rxn where all of the
  acidic protons leave the acid.

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Buffers

• Buffer soln in which pH remains relatively
  constant when small amounts of acid or base are
  added.
• Ex. blood is a buffer
• A buffer works due to the presence of a conjugate
  acid-base pair in the soln.
• A buffer is made from a weak acid and its
  conjugate base or a weak base and its conjugate
  acid.

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• A buffer has a special equilibrium
• H2CO3 H2O ? H3O HCO3-
• Acid conj. Base

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Buffers resist changes in pH. A key buffer
solution in our body consists of carbonic acid
(H2CO3) and Sodium Bicarbonate (NaHCO3).
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Body pH 7.35-7.45
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Kidneys play an important role in maintaining
a constant pH. They excrete excess acidic or
basic substances. They are a back-up
neutralization process. If the blood is too
acidic, they produce weak bases. If the blood is
too basic, they produce weak acids.
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Acidosis- too much acid in the system pH below
7.35

• Results lungs fail to expel CO2, mental
  confusion, disorientation, shock, coma, death.

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Alkalosis- too much base in the system pH above
7.45

• Results Severe vomiting of HCl from the stomach,
  irregular breathing and muscle contractions,
  convulsions, death.

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Acid Rain

• Acid rain occurs from emissions of SOxand NOx
  compounds. These emissions come from
  automobiles, power plants, and industry.
• The SOx mixes with water vapor in the atmosphere
  to produce sulfurous and sulfuric acid.
• SO3 H2O ? H2SO4

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• NOx combines with water vapor to make nitric
  acid.
• Acid rain increases the acidity of soil which
  adversely affects plant life.
• Acid rain increases the acidity of rivers,
  streams, and lakes which can harm or kill aquatic
  life.

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Acid rain also erodes statues and buildings
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Acid Dissociation Constant Ka

• Ka can be used to determine the degree to which
  an acid will dissociate.
• The higher the value of Ka the stronger the acid.
• Of the following, which is the strongest?
• HA Ka 1.0 x 10-5
• HB Ka 1.0 x 10-2

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• Anhydrides some oxides can become acids or
  bases by adding the elements contained in water.
• Oxides of nonmetals (like C, S, N) produce acidic
  soln.
• Oxides of metals usually form basic soln.
• CO2(g) H2O(l) ? H2CO3(aq)

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• CaO(s) H2O(l) ? Ca2(aq) 2OH-(l)
• SO3 is the anhydride of H2SO4
• MgO is the anhydride of Mg(OH)2
• What acid will form from N2O5?
• HNO3

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pH

• Ion Product Constant for Water (Kw)
• 2H2O(l) ? H3O(aq) OH-(aq)
• Keq H3OOH-
• Keq H2O Kw H3OOH-
• Kw H3OOH- (1x10-7) (1x10-7)
• Kw 1x10-14