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Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2

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Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2. Generally, “ x is small” will work when both of these are true: initial [ ]’s of acid & salt are K a is _______________ Most problems: initial acid & salt [ ]’s should be _____ to ______ > value of K a.

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slide1

Chemistry 223 Chapter 21:

Buffers and the Titration of

Acids and Bases PART 2

slide2

Generally, “x is small” will work when both of these are true:

    • initial [ ]’s of acid & salt are
    • Ka is _______________
  • Most problems: initial acid & salt [ ]’s should be _____ to ______ > value of Ka
how much does the ph of a buffer change when an acid or base is added
How Much Does the pH of a Buffer Change When an Acid or Base Is Added?
  • Calculating new pH after adding acid or base requires doing 2 calculations:
slide4

stoichiometry calculation for added chemical with HA or A- of buffer to reduce initial [ ] and increase [ ] of the other

    • added acid reacts with
    • added base reacts with
slide6
What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it?

HC2H3O2 + OH− C2H3O2 + H2O

0 100 mol hc 2 h 3 o 2 and 0 100 mol nac 2 h 3 o 2 in 1 00 l with 0 010 mol naoh added
0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added.

HC2H3O2 + OH− C2H3O2 + H2O

0 100 mol hc 2 h 3 o 2 and 0 100 mol nac 2 h 3 o 2 in 1 00 l with 0 010 mol naoh added1
0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added.

HC2H3O2 + OH− C2H3O2 + H2O

−0.010

+0.010

−0.010

0.110

0

0.090

0.090

0.110

0

slide12

HC2H3O2 + H2O  C2H3O2 + H3O+

x

+x

+x

x

0.090 x

0.110 + x

slide16

0.090 x

x

0.110 + x

x = 1.47 x 10−5

slide19

Example: What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it?

HC2H3O2 + H2O  C2H3O2 + H3O+

slide20

HC2H3O2 + H2O  C2H3O2 + H3O+

Ka for HC2H3O2 = 1.8 x 10−5

Tro: Chemistry: A Molecular Approach, 2/e

slide21

HC2H3O2 + H2O  C2H3O2 + H3O+

pKa for HC2H3O2 = 4.745

slide22

Compare effect on pH of adding 0.010 molNaOH to a 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 buffer in 1.00 L to adding 0.010 molNaOH to 1.00 L of pure water

HC2H3O2 + H2O  C2H3O2 + H3O+

pKa for HC2H3O2 = 4.745

slide24

Clicker question: What is the pH of a buffer that has 0.140 moles HF (pKa = 3.15) and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added? (The “x is small” approximation is valid)

slide25
What is the pH of a buffer that has 0.140 moles HF and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?
slide28

F− + H3O+  HF + H2O

−0.020

−0.020

+0.020

henderson hasselbalch equation for basic buffers
Henderson-Hasselbalch Equation for Basic Buffers
  • chemical equation of a basic buffer is written with a weak base as a reactant and its conjugate acid as a product
slide31

B: + H2O  H:B+ + OH−

chemical eqtn of basic buffer must be looked at like an acid reaction

this does not affect [ ]’s,

just the way we are looking at the rxtn

relationship between p k a and p k b
Relationship between pKa and pKb
  • relationship btwn Ka of a weak acid and Kb of its conjugate base,
  • also a relationship btwn pKa of a weak acid and pKb of its conjugate base
what is ph of a buffer that is 0 50 m nh 3 p k b 4 75 and 0 20 m nh 4 cl
What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl?

NH3 + H2O  NH4+ + OH−

henderson hasselbalch equation for basic buffers1
Henderson-Hasselbalch Equation for Basic Buffers
  • chemeqtn of basic buffer is written with weak base as a reactant and its conjugate acid as a product
  • B: + H2O  H:B+ + OH−
henderson hasselbalch equation for basic buffers2
Henderson-Hasselbalch Equation for Basic Buffers
  • We can rewrite Henderson-Hasselbalch eqtn for chem eqtn of basic buffer in terms of __________
what is ph of a buffer that is 0 50 m nh 3 p k b 4 75 and 0 20 m nh 4 cl1
What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl?

NH3 + H2O  NH4+ + OH−

slide37

Polyprotic Acids & Bases

Polyprotic acids contain > one ionizable p+

and the protons are lost in a stepwise manner.

Fully protonated species is always the strongest acid.

Why?

slide38

Strong acids have

very weak conjugate bases

Strong bases have

very weak conjugate acids

slide39

Titrations of Polyprotic Acids or Bases

When strong base is added to soltn of a polyprotic acid –

neutralization rxtn occurs in stages.

Most acidic group titrated 1st,

followed by next most acidic group.

slide40

Titrations of Polyprotic Acids or Bases

If pKa values are separated by at least 3 pKa units,

then overall titration curve shows

well-resolved “steps”

corresponding to titration of each p+.

slide43

Indicators

Most acid-base titrations are not monitored by recording pH

as a function of

amount of strong acid or base soltn

used as a titrant

slide44

Indicators

Instead, acid-base indicator is used,

Compounds that D color at a particular pH

slide45

If carefully selected, undergo dramatic color D

at pH corresponding to ________________ of titration

slide46

Indicators

Acid-base indicators are typically weak acids or bases.

Color D’scorrespond to

_____________________________

of the indicator itself.

slide47

Indicators

Chemistry of indicators general equation:

Protonated form =

conjugate base (deprotonated) =

slide48

Indicators

ionization constant for deprotonation of indicator Hn:

Kin = [H+] [n–] / [Hn]

pKindetermines pH at which indicator D’s color

slide49

Properties of Good Indicators

  • Color change must be easily detected
  • Color change must be rapid
  • Indicator must not react with substance being titrated
  • Indicator should have pKin within
slide50

Choosing a Good Indicator

for Acid/Base Titration

For titrations of

strong acids and strong bases

(and vice versa),

any indicator with a pKin

slide51

Choosing a Good Indicator

for Acid/Base Titration

For titration of

a weak acid,

pH at equivalence pt whichis >7,

indicator such as

phenolphthalein or thymol blue,

(pKin > 7)

should be used.

slide52

Choosing a Good Indicator

for Acid/Base Titration

For titration of

a weak base,

pH at equivalence pt is < 7,

an indicator like methyl red or bromcresol blue,

(pKin < 7),

should be used.

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