I just need help with a few questions from my lab. Thanks! Lab Notes: Experiment
ID: 499932 • Letter: I
Question
I just need help with a few questions from my lab. Thanks!
Lab Notes:
Experiment 1
pH Test Tube 1 (10 mL water): 7.00
pH Test Tube 2 (10 mL acetone): 7.00
pH Test Tube 3 (10 mL 5M citric acid): 1.22
pH Test Tube 4 (10 mL 5% vinegar): 2.39
pH Test Tube 5 (10 mL 4M ammonia): 11.93
pH Test Tube 6 (10 mL diluted bleach): 10.67
After adding 2 drops of Bromothymol Blue to each test tube:
pH Test Tube 1 (10 mL water): 7.00
Color of Test Tube 1: Green
pH Test Tube 2 (10 mL acetone): 7.00
Color of Test Tube 2: Green
pH Test Tube 3 (10 mL 5M citric acid): 1.22
Color of Test Tube 3: Yellow
pH Test Tube 4 (10 mL 5% vinegar): 2.40
Color of Test Tube 4: Yellow
pH Test Tube 5 (10 mL 4M ammonia): 11.93
Color of Test Tube 5: Blue
pH Test Tube 6 (10 mL diluted bleach): 10.67
Color of Test Tube 6: Blue
Only 3 colors changes, the pH was not affected by adding the drops, except a 0.01 increase in pH for Test Tube 4.
Experiment 2
Part 1:
pH Test Tube 1 (5 mL water): 7.00
pH Test Tube 2 (5 mL Sodium Monobasic Phosphate): 4.68
pH Test Tube 3 (5 mL Sodium Dibasic Phosphate): 9.72
pH Test Tube 4 (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.21
After adding 2 drops of 0.5 M Hydrochloric Acid:
pH Test Tube 1 (5 mL water): 2.01
pH Test Tube 2 (5 mL Sodium Monobasic Phosphate): 3.12
pH Test Tube 3 (5 mL Sodium Dibasic Phosphate): 8.16
pH Test Tube 4 (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.03
Part 2:
pH Test Tube 1 (5 mL water): 7.00
pH Test Tube 2 (5 mL Sodium Monobasic Phosphate): 4.68
pH Test Tube 3 (5 mL Sodium Dibasic Phosphate): 9.72
pH Test Tube 4 (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.21
After adding 2 drops of 0.5 M Sodium Hydroxide:
pH Test Tube 1 (5 mL water): 11.99
pH Test Tube 2 (5 mL Sodium Monobasic Phosphate): 6.25
pH Test Tube 3 (5 mL Sodium Dibasic Phosphate): 11.29
pH Test Tube 4 (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.38
Experiment 3
Part 1:
pH Test Tube (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.21
pH Test Tube after 1 drop of 0.5M Hydrochloric Acid: 7.12
pH Test Tube after 2 drops of 0.5M Hydrochloric Acid: 7.03
pH Test Tube after 3 drops of 0.5M Hydrochloric Acid: 6.94
pH Test Tube after 4 drops of 0.5M Hydrochloric Acid: 6.84
pH Test Tube after 5 drops of 0.5M Hydrochloric Acid: 6.73
pH Test Tube after 6 drops of 0.5M Hydrochloric Acid: 6.60
pH Test Tube after 7 drops of 0.5M Hydrochloric Acid: 6.45
pH Test Tube after 8 drops of 0.5M Hydrochloric Acid: 6.25
pH Test Tube after 9 drops of 0.5M Hydrochloric Acid: 5.93
pH Test Tube after 10 drops of 0.5M Hydrochloric Acid: 4.68
pH Test Tube after 11 drops of 0.5M Hydrochloric Acid: 3.44
pH Test Tube after 12 drops of 0.5M Hydrochloric Acid: 3.12
pH Test Tube after 13 drops of 0.5M Hydrochloric Acid: 2.93
Took 13 drops for pH to drop below 3.0
Part 2:
pH Test Tube (2.5 mL Sodium Monobasic Phosphate & 2.5 mL Sodium Dibasic Phosphate): 7.21
pH Test Tube after 1 drop of 0.5M Sodium Hydroxide: 7.29
pH Test Tube after 2 drops of 0.5M Sodium Hydroxide: 7.38
pH Test Tube after 3 drops of 0.5M Sodium Hydroxide: 7.47
pH Test Tube after 4 drops of 0.5M Sodium Hydroxide: 7.57
pH Test Tube after 5 drops of 0.5M Sodium Hydroxide: 7.68
pH Test Tube after 6 drops of 0.5M Sodium Hydroxide: 7.81
pH Test Tube after 7 drops of 0.5M Sodium Hydroxide: 7.96
pH Test Tube after 8 drops of 0.5M Sodium Hydroxide: 8.16
pH Test Tube after 9 drops of 0.5M Sodium Hydroxide: 8.48
pH Test Tube after 10 drops of 0.5M Sodium Hydroxide: 9.72
pH Test Tube after 11 drops of 0.5M Sodium Hydroxide: 10.95
pH Test Tube after 12 drops of 0.5M Sodium Hydroxide: 11.28
pH Test Tube after 13 drops of 0.5M Sodium Hydroxide: 11.48
pH Test Tube after 14 drops of 0.5M Sodium Hydroxide: 11.63
pH Test Tube after 15 drops of 0.5M Sodium Hydroxide: 11.75
Questions:
Experiment 2
1. Which of the four solutions tested is the best buffer against changes in pH caused by the addition of acid or base?
Experiment 3
2. Suppose you forget to write down the contents of four test tubes. Each contains a different solution that has a different pH. What is the best way to quickly identify the contents of each test tube
A. add hydrocholoric acid to each solution, then check the pH with a meter
B. add sodium hydroxide to each solution, then check the pH with a meter
C. check the pH of each solution with thymol blue
D. check the pH of each solution with a pH meter
3. Explain why phosphate buffer needs both Na2HPO4 and NaH2PO4 in order to resist changes in pH.
4. Which chemical provides the conjugate base in the buffer containing NaH2PO4 and Na2HPO4?
5. Your lab mate attempts to use bromothymol blue to differentiate between two solutions – one that should be pH 7.3, and another that should be pH 6.7. What is your advice to your lab mate? Do you agree with his decision?
6. Predict what might happen if you made up phosphate buffer with only half as much NaH2PO4 compared to Na2HPO4.
Experiment 2
1. Which of the four solutions tested is the best buffer against changes in pH caused by the addition of acid or base?
Explanation / Answer
Answer to Q1)
A buffer is a solution that resists to change in pH when acid or base is added to it.
For Solution 1, the change in pH when HCl was added is of (7.00 -2.01) 4.99 while the change in pH when NaOH was added is of (11.99 - 7.00) 4.99
For Solution 2, the change in pH when HCl was added is of 1.56 while the change in pH when NaOH was added is of 1.57
For Solution 3, the change in pH when HCl was added is of 1.56 while the change in pH when NaOH was added is of 1.57
For Solution 4, the change in pH when HCl was added is of 0.18 while the change in pH when NaOH was added is of 0.17. Thus this solution shows least change in pH and hence a 1:1 v/v mixture of Sodium Monobasic Phosphate and Sodium Dibasic Phosphate is a very good buffer.
Answer to Q2)
Bromothymol blue can tell you whether the solution is acidic, neutral or basic. In experiment 3, there are subtle changes in pH and hence you will need a pH meter. Use of pH meter involves calibrating the instrument with pH 7.0 and 4.0 solutions (once in 15 days to a month) and washing the probe to avoid cross contamination and to ensure accuracy of the reading. Though accurate it is a time consuming process when compared to use of Bromothymol blue. If you add acid or base to the solution, the pH will change. Hence, you should directly check the pH of the four test tubes and compare it with the pH values already recorded. Note, the pH decreases gradually when HCl is added and increases gradually when NaOH is added. Thus you can number the missing test tubes easily.
Answer to Q3)
The dissociation of NaH2PO4is given as:
H2PO4- <==> HPO4-2 + H+
Using Henderson Hesselbalch equation:
pH = pKa + log [HPO4-2] / [H2PO4-]
A solution is a good buffer when pH = pKa
Thus log [HPO4-2] / [H2PO4-] = 0
[HPO4-2] = [H2PO4-]
In other words, if H+ ions are added to this buffer solution then HPO4-2 will utilise them and convert into H2PO4- and if OH- are added to this buffer solution then H2PO4- will utilise them and convert into H+ and HPO4-2. This will resist the changes in pH making the 1:1 v/v mixture of Sodium Monobasic Phosphate and Sodium Dibasic Phosphate is a very good buffer.