Buffers and Titrations

The Common Ion Effect & Buffer Solutions

• __________________- solutions in which the same ion is produced by two different compounds

• __________________- resist changes in pH when acids or bases are added to them

– due to common ion effect

2

The Common Ion Effect & Buffer Solutions

Two common kinds of buffer solutions

1 solutions of a ___________plus a soluble ______________________________

2 solutions of a ___________ plus a soluble ______________________________

3

Weak Acids plus Salts of Weak Acids

For example ~ acetic acid CH3COOH and sodium acetate NaCH3COO

reacts with base

CH COOH + H O H O CH COO

Na CH COO Na CH COO

reacts with acid

3 2 3+

3-

3 3-

⇓→ +←

→ +

⇑

+ − ≈ +100%

4

Ex. 1) Calculate the concentration of H+ and the pH of a solution that is 0.15 M in acetic acid and 0.15 M in sodium acetate. Ka = 1.8 x 10-5

*Always start the reaction with your weak acid (or weak base) added to water.

(note: sodium acetate completely dissociates)

R

I.

C.

E. 5

Ex. 1) Calculate the concentration of H+ and the pH of a solution that is 0.15 M in acetic acid and 0.15 M in sodium acetate. Ka = 1.8 x 10-5

R CH3COOH + H2O CH3COO- + H3O+

I. 0.15 0.15 0

C. -x +x +x

E. 0.15 – x 0.15 + x x

6

[ ][ ][ ]

( )( )( )x

xx−+

=×== −15.0

15.0108.1COOHCH

COOCHHK 53

-3

+

a

( ) ( )

[ ]74.4pH

H108.1

108.10.15

0.15gives assumption thisMaking

15.015.0 and 15.015.0

5

5

==×=

×=

≈−≈+

+−

−

Mx

x

xx

7

Compare the acidity of a pure acetic acid solution and the buffer we just described.

Notice that [H+] is _____ times greater in pure acetic acid than in buffer solution.

Solution [H+] pH 0.15 M CH3COOH 1.6 x 10-3 M 2.80 0.15 M CH3COOH

& 0.15 M NaCH3COO

1.8 x 10-5 M 4.74

8

Solution

[H+]

pH

0.15 M CH3COOH

1.6 x 10-3 M

2.80

0.15 M CH3COOH

&

0.15 M NaCH3COO

1.8 x 10-5 M

4.74

Weak Bases plus Salts of Weak Bases

Ex.2) Calculate the concentration of OH- and the pH of the solution that is 0.15 M in aqueous ammonia, NH3, and 0.30 M in ammonium nitrate, NH4NO3. Kb = 1.8 x 10-5

R

I

C

E9

Weak Bases plus Salts of Weak Bases

Ex.2) Calculate the concentration of OH- and the pH of the solution that is 0.15 M in aqueous ammonia, NH3, and 0.30 M in ammonium nitrate, NH4NO3. Kb = 1.8 x 10-5

R NH3 + H2O NH4+ + OH-

I 0.15 0.30 0

C -x + x + x

E 0.15 –x 0.30 + x x10

Substitute these values into the ionization expression for ammonia and solve algebraically.

[ ][ ][ ]

( )( )( )

( )( )( )

[ ]8.95pH and 5.05pOHOH100.9

108.115.0

30.0K

assumption apply the

108.115.0

30.0K

108.1NH

OHNHK

6

5b

5b

5

3

4b

===×=

×==

×=−

+=

×==

−−

−

−

−−+

Mx

x

xxx

11

Weak Bases plus Salts of Weak Bases

Let’s compare the aqueous ammonia concentration to that of the buffer described above.

Note, the [OH-] in aqueous ammonia is ____times greater than in the buffer.

Solution [OH-] pH 0.15 M NH3 1.6 x 10-3 M 11.20 0.15 M NH3

& 0.30 M NH4NO3

9.0 x 10-6 M 8.95

12

Solution

[OH-]

pH

0.15 M NH3

1.6 x 10-3 M

11.20

0.15 M NH3

&

0.30 M NH4NO3

9.0 x 10-6 M

8.95

Henderson-Hasselbach equation

On your green sheets

pH = pKa + log [A-][HA]

Takes into account the dissociation factor of the weak acid (or weak base)

Henderson-Hasselbach: an easier way to look at the eqn.

[ ][ ]acidsaltlogpKpH a +=

[ ][ ]basesaltlogpKpOH b +=

For acids:

For bases:

Remember: pX = -log X

Buffering Action Buffer solutions resist changes in pH.

Ex. 3) If 0.020 mole of HCl is added to 1.00 liter of solution that is 0.100 M in aqueous ammonia and 0.200 M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the gaseous HCl.

You must decide what the HCl will react with…ammonia or ammonium?

15

1st ~ Calculate the pH of the original buffer solution to find the initial pH

2nd ~ Calculate the concentration of all species after the addition of HCl. HCl will react with some of the ammonia

17

3rd ~ Now that you have the concentrations of our salt and base, you can calculate the new pH.

18

4th ~ Calculate the change in pH.

19

Ex. 4) If 0.020 mole of NaOH is added to 1.00 liter of solution that is 0.100 M in aqueous ammonia and 0.200 M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the solid NaOH.

(Does the NaOH react with the ammonia or the ammonium?)

20

Preparation of Buffer SolutionsEx. 5) Calculate the concentration of H+ and the pH of

the solution prepared by mixing 200 mL of 0.150 Macetic acid and 100 mL of 0.100 M sodium hydroxide solutions.

Determine the molar amounts of acetic acid and sodium hydroxide (before reaction)

One of the reactants will be the limiting reagent, one will be in excess.

21

Preparation of Buffer Solutions For ______________ situations, it is sometimes

important to prepare a buffer solution of a given pH.

Ex. 6) A) Find the number of moles of solid ammonium chloride, NH4Cl, that must be used to prepare 1.00 L of a buffer solution that is 0.10 M in aqueous ammonia, and that has a pH of 9.15

B) What mass is needed?

22

Acid-Base Indicators ______________ - point at which chemically

equivalent amounts of acid and base have reacted

______________ - point at which chemical indicator changes color

23

Common Acid-Base Indicators

Indicator

Color in acidic range

pH range

Color in basic range

Methyl violet

yellow 0-2 purple

Methyl orange

pink 3.1-4.4 yellow

Litmus red 4.7-8.2 blue Phenol-

phthalein colorless 8.3-10.0 red

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Indicator

Color in acidic range

pH range

Color in

basic range

Methyl violet

yellow

0-2

purple

Methyl orange

pink

3.1-4.4

yellow

Litmus

red

4.7-8.2

blue

Phenol-

phthalein

colorless

8.3-10.0

red

Strong Acid/Strong Base Titration Curves

______________ ________are graphs that show the pH at various amounts of titrate added. Allows you to find the ______________ ________.

For Titration curves, Plot ______________ of acid or base added in titration.

25

Ex. 7) Consider the titration of 100.0 mL of 0.100 M perchloric acid with 0.100 M potassium hydroxide. Find the equivalence point of this rxn.Plot pH vs. mL of KOH added1:1 mole ratio

OHKClOKOHHClO 244 +→+

Strong Acid/Strong Base Titration Curves

Before titration starts the pH of the HClO4 solution is _______ Remember that perchloric acid is a strong acid

[ ]000.1log(0.100)pH

100.0H10000.100 100.0ClOHHClO 4

%1004

=−==

+ →

+

−+≈

M M.M M

27

After 20.0 mL of 0.100 M KOH has been added the new pH is _______.

28

After 50.0 mL of 0.100 M KOH has been added the pH is _______.

29

After 90.0 mL of 0.100 M KOH has been added the pH is _______.

30

After 100.0 mL of 0.100 M KOH has been added the pH is _______.

31

Strong Acid/Strong Base Titration Curves

We’ve calculated only a few points on the titration curve. Similar calculations for the remainder of titration can show clearly the _______ of the titration curve.

32

Weak Acid/Strong Base Titration Curves

Salts of weak acids and strong bases hydrolyze to give basic solns so the soln is _______ at the equivalence point and the soln is _____________ before the ____________ point.

33

Strong Acid/Weak BaseTitration Curves

Titration curves for Strong Acid/Weak Bases look similar to Strong Base/Weak Acid but they are inverted. The soln is _________before the ______________and is ______________ at the _____________________

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Weak Acid/Weak BaseTitration Curves

Titration curves have ______________vertical sections.

Solution is buffered both _______ and _______the equivalence point.

____________________cannot be used. Instead you can measure the ______________ in order to find the end point.

The math is complex, we will not worry about it in AP Chem.

35

More Fun Chemistry for you Blood is slightly basic, having a pH of 7.35 to 7.45. What

chemical species causes our blood to be basic? How does our body regulate the pH of blood?

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Buffers and TitrationsThe Common Ion Effect & Buffer SolutionsThe Common Ion Effect & Buffer SolutionsWeak Acids plus Salts of Weak AcidsSlide Number 5Slide Number 6Slide Number 7Slide Number 8Weak Bases plus Salts of Weak BasesWeak Bases plus Salts of Weak BasesSlide Number 11Weak Bases plus Salts of Weak BasesHenderson-Hasselbach equation�Henderson-Hasselbach: �an easier way to look at the eqn.�Buffering ActionSlide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Preparation of Buffer SolutionsPreparation of Buffer SolutionsAcid-Base IndicatorsCommon Acid-Base IndicatorsStrong Acid/Strong Base �Titration CurvesSlide Number 26Strong Acid/Strong Base �Titration CurvesSlide Number 28Slide Number 29Slide Number 30Slide Number 31Strong Acid/Strong Base �Titration CurvesWeak Acid/Strong Base �Titration CurvesStrong Acid/Weak Base�Titration CurvesWeak Acid/Weak Base�Titration CurvesMore Fun Chemistry for you