Chem1151 lab12 - lab 12 PDF

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lab 12...

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Acids and Bases PRE-LAB QUESTIONS 1. Where do water and hydrochloric acid (HCl) lie on the pH scale in relation to each other?

The pH of water is 7 (neutral) and the pH of HCl is 3.01(acidic).

2. Give an example of a buffer, and describe what it is used for.

An example of a buffer solution is bicarbonate in the blood, which maintains the body's internal pH by balancing the amount of carbonic acid (H2CO3), bicarbonate ion (HCO−), and carbon dioxide (CO2) in the blood, duodenum, other tissues, to support proper metabolic function.

3. Explain why conjugate pairs can be composed of weak acids and strong bases or weak bases and strong acids.

A strong acid or base is unstable and will dissociate when added to water. A weak acid or base is stable and won’t dissociate in water. If an acid is to be "strong", it must readily dissociate to release hydrogen ions. However, the conjugate base that is formed must also be stable, so that it does not combine back with the released hydrogen ions, making it a weak base.

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Acids and Bases EXPERIMENT 1: PREPARING A BUFFER Data Sheet Table 1. Sodium Acetate Data Sodium Acetate (g)

Molarity of Sodium Acetate (M)

5g

0.061 M

Table 2. Initial Buffer Solution and pH Readings Buffer

Volume of Acetic Acid (mL)

Volume of Sodium Acetate (mL)

pH

A

8

8

2.48

Table 3. Buffer 1 Solutions and pH Readings Buffer

Volume of Buffer Solution A (mL)

Volume of Acetic Acid (mL)

pH

B (1)

2

8

1.93

C (1)

4

8

2.09

Table 4. Buffer 2 Solutions and pH Readings Buffer

Volume of Buffer Solution A (mL)

Volume of Sodium Acetate (mL)

pH

B (2)

2

8

2.11

C (2)

4

8

2.22

Photo Documentation Paste photos from steps 12 and 15 below:

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Acids and Bases steps 12

steps 15

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Acids and Bases

Experiment 1 Post-Lab Questions 1. Show your calculations for the molarity of sodium acetate (recorded in Table 1).

Molarity is the moles of solute dissolved per liter of solvent.

5 g of sodium acetate x 1 mol of sodium acetate 82.034 g of sodium acetate

= 0.061 mol of sodium acetate

Molarity = moles of solute = 0.061 mol of sodium acetate = 0.847 M Liters of solution 0.072 L of solution

4. Using the Henderson-Hasselbalch equation, calculate the predicted pH value for each buffer solution based on the volumes given in Tables 2, 3, and 4. The pKa of acetic acid is 4.75. Show your work.

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Acids and Bases Henderson-Hasselbalch equation: pH = pKa + log [base / acid] a. Buffer A: pH = 4.75 + log [8 / 8] = 4.75 b. Buffer B (1): pH = 4.75 + log [2 / 8] = 4.15

c. Buffer C (1): pH = 4.75 + log [4 / 8] = 4.44

d. Buffer B (2): pH = 4.75 + log [2 / 8] = 4.15 e. Buffer C (2): pH = 4.75 + log [4 / 8] = 4.44 5. Calculate your pH percent error for each buffer solution based on your theoretical calculations in Question 2 and your measured values in Table 4. Show your work. % error = [predicted value – experimental value] x100 Predicted value a. Buffer A: (4.75-2.48)/4.75 x 100 = 47.8%

b. Buffer B (1): (4.15-1.93)/4.15 x 100 = 46.7%

c. Buffer C (1): (4.44-2.09)/4.44 x 100 = 49.5%

d. Buffer B (2): (4.15-2.11)/4.15 x 100 = 42.9%

e. Buffer C (2): (4.44-2.22)/4.44 x 100 = 46.7%

6. What are some potential sources of error in this experiment that could account for your percent error values?

The pH meter was not working correctly.

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Acids and Bases

EXPERIMENT 2: EXPLORING BUFFERING CAPACITY Data Sheet Table 5. Observing the Effects of Acid on Buffer F and Buffer H (water) Buffer

pH with 0 mL of Cola

pH with 5 mL of Cola

pH with 10 mL of Cola

pH with 20 mL of Cola

Buffer F

2.56

2.31

2.30

2.24

Buffer H (water)

2.66

1.04

n/a

n/a

Table 6. Observing the Effects of Base on Buffer G and Buffer I (water) Buffer

pH with 0 mL of Borax Solution

pH with 5 mL of Borax Solution

pH with 10 mL of Borax Solution

pH with 20 mL of Borax Solution

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Acids and Bases Buffer G

2.56

Buffer I (water)

2.76

2.46 2.74

2.57

2.81

n/a

n/a

Photo Documentation Paste photos of steps 6, 9, 10, and 13 from part 1 and steps 9, 13, and 15 from part 2 below

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Acids and Bases

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Acids and Bases

©2019, eScience Labs

Acids and Bases

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Acids and Bases

Experiment 2 Post-Lab Questions 1. What happened when the cola and borax solutions were added to the water?

When cola was added to the water, the solution became acidic and when Borax was added, it became basic.

7. Is water a good buffering system? Explain your scientific reasoning.

A buffer system is a solution that can resist a change in pH when acids or bases are added to it. In the experiment, it showed that when cola and borax are added to water, the pH changes which is why water is not a good buffering system.

8. How would the pH change if you added 40 mL of the borax solution to 40 mL of water? Explain your reasoning.

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Acids and Bases When 40 mL of the borax solution is added to 40 mL of water, the pH of the solution will increase making it basic. Borax is a compound that can dissociate in solution producing sodium hydroxide which makes the solution basic. This reaction is represented using the equation below.

Na2B4O7+7 H2O → 4 H3BO3 + 2 NaOH

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