Pick Up a Clicker!

Schedule8:10-9:00 Questions9:00-9:45 Test on Acids and Bases9:45-12:00 Buffers and titration curves

Good Morning

Chapter 15 – Titration Curves

2

Titration

• Strong acid/strong base• Strong acid/weak base• Strong base/weak acid

3

Setup to Do pH Titration of an Acid or Base

Photo © Brooks/Cole, Cengage Learning Company. All rights reserved.

4

Titration of 50ml of 0.20M HNO3 with 0.100M NaOH: No NaOH Added

Of solution in the beaker (called analyte)

From the burette, called the titrant

5

Titration of 50ml of 0.20M HNO3: 10.0 mL of 0.100 M NaOH Added

6

Titration of 50ml of 0.20M HNO3: 50.0 mL (total) of 0.100 M NaOH Added

7

Titration of 50ml of 0.20M HNO3: 100.0 mL (total) of 0.100 M NaOH Added

8

Titration of 50ml of 0.20M HNO3: 150.0 mL (total) of 0.100 M NaOH Added

9

Titration of 50ml of 0.20M HNO3: 200 mL (total) of 0.100 M NaOH Added

10

Titration of 50ml of 0.20M HNO3 with 0.100 M NaOH

Note: at the equivalence point the number of moles of OH¯ added = the number of moles of H3O+ titrated

The terms equivalence point and end point mean the same thing.

11

Strong base/weak acid titration

Unlike the titration of a strong acid, the pH of a weak base is buffered at pH valued of pKa ± 1.

][

][log

acid

basepKpH a

12

][

][log

acid

basepKpH a

The endpoint of the titration is at 50 mL of added OH¯. The midpoint is halfway, at 25 ml of added OH¯.

At the midpoint, pH = pKa.

In other words, the pKa can be read directly from the titration curve.

13

Below is shown the titration of a weak base with HClO4.

For this buffer it shows how the pH changes with added strong acid.

It can be thought of as a buffer profile.

14

Each point in this titration is the result of a separate experiment.

Point by point is shown buffer pH for a given buffer composition. In sum, it shows how the pH changes with added strong acid.

It gives you the pKa of the weak acid/base buffer pair.

15

What percent of the NH3 has been converted to NH4+ after

25ml of HClO4 have been added?

Titration of 100 ml of 0.05M NH3

NH3 + H3O+ NH4+ + H2O

16

What is the pKa of the conjugate weak acid? a. 11 b. 3 c. 9.3 d. 4.7 e. lost

][

][log

acid

basepKpH a

17

Titration of 100 ml of 0.05M NH3

What percent of the NH3 has been converted to NH4+ after

20ml of HClO4 has been added?

After 40ml of HClO4 has been added?After 50 ml?

18

What are the major species in solution at the end of the

titration?

a. ClO4¯, NH3, H3O+ b. HClO4, NH3 c. ClO4¯, NH4

+, H3O+ d. HClO4, NH4

+

19

If the volume of the analyte NH3 is 100 mL, what was its

concentration at the start of the titration?

a. 0.10M b. 0.20M c. 0.05M d. Lost!

20

Titration of 100 ml of 0.05M NH3

What is buffering region here, in terms of mL HClO4 added?

Does a buffering range of pH = pKa ± 1 seem about right?

21

How would you calculate the pH at the endpoint of this titration of 100 ml of 0.05M NH3?

Hint: first you need to know what will react with water, and its concentration!

22

What is the pH at the midpoint of a HOCl titration (Ka = 3.5x10-8)1. 7.52. 6.53. Lost

What is the pH at the midpoint of a OCl¯ titration (Kb = 2.9x10-7)4. 7.55. 6.56. lost

Consider the titration of 0.20M HOCl. What is the concentration of OCl¯ at the midpoint of the titration (ignoring dilution)?1. 0.20M2. 0.10M3. Not possible to determine without more information4. lost

23

Titration of a polyprotic acid: H3PO4

• pKa values• Species at 5ml

intervals• pH of

amphoteric salts

H3PO4

H2PO4¯

HPO4-2

PO4-3

pKa1 = 2.1 pKa2 = 7.2 pKa3 = 12.3

24

How to prepare a buffer

25

How to prepare a bufferSuppose you wanted to make a pH 7.4 phosphate buffer

Given that pKa1 = 2.12, pKa2 = 7.21, and pKa3 =12.3, what chemicals would you use from this list:H3PO4, KH2PO4, K2HPO4, K3PO4

What would be the ratio of Base/Acid that you would use?

Suppose you wanted a 0.20M, pH 7.4 phosphate buffer. How much base and how much conjugate acid would you use to make 1 liter of buffer?

KH2PO4, K2HPO4

][

][log2.74.7

acid

base

26

The steps for a buffer preparation calculation:

1. Determine the weak acid/weak base couple using the pH of the buffer as a guide

2. Determine the ratio of base to acid in the buffer using pH = pka + log(B/A)

3. Determine the actual concentrations of B and A to be used based on the desired buffer concentration and the B/A ratio.

27

Example: Make a 0.15M pH 6.7 carbonate buffer.

1. Determine the weak acid/weak base couple using the pH of the buffer as a guide

For H2CO3, pKa1 = 6.4, pKa2 = 10.3

Choose the weak acid with pKa1 = 6.4, and its conjugate base: H2CO3/HCO3¯

28

Example: Make a 0.15M pH 6.7 carbonate buffer.

2. Determine the ratio of base to acid in the buffer using pH = pka + log(B/A)

6.7 = 6.4 + log([HCO3¯]/[H2CO3])

[HCO3¯]/[H2CO3] = 2.0

29

Example: Make a 0.15M pH 6.7 carbonate buffer.

3. Determine the actual concentrations of B and A to be used based on the desired buffer concentration and the B/A ratio.

[HCO3¯]/[H2CO3] = 2.0

[HCO3¯] + [H2CO3] = 0.15

Solve these two equations for the two unknowns:

[HCO3¯] = 0.10M [H2CO3] = 0.05M 30

Try a problem: What would you need in solution to have a 0.30M, pH 7.1, hypochlorous buffer?HClO: pKa= 7.41. Determine the weak acid/weak base couple

using the pH of the buffer as a guide2. Determine the ratio of base to acid in the buffer

using pH = pka + log(B/A)3. Determine the actual concentrations of B and A

to be used based on the desired buffer concentration and the B/A ratio.

31

Could you prepare this buffer using only HClO and KOH?

How many moles of HClO would you use to prepare 1.0L of 0.30M, pH 7 buffer?1. 0.12. 0.23. 0.34. lost

About how many moles KOH would you expect to need to make 1.0L the buffer?5. 0.16. 0.27. 0.38. lost

32