I just need help with a few questions from my lab. Thanks! Lab Notes: Experiment
ID: 499285 • 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 1:
1. The strong acid and strong base used in this lab are the same concentration. Thus, the magnitude of the pH change caused by addition of a single drop will be the same on either side of the starting pH. Addition of acid lessens the pH, and addition of base raises the pH. In the graph below, negative drops indicate the magnitude of drops of acid, whereas positive drops indicate the magnitude of drops of base.
What trends in the buffer's ability to maintain a constant pH do you observe in the graph?
A. Initially, the buffer was not able to stabilize the pH and the graph changed dramatically. However, after an initial change, the buffer was able to draw the pH closer to neutral number.
B. The buffer was able to keep pH changes small for a large number of drops, but past a critical point the pH changed dramatically.
C. The buffer kept the change in pH moderate no matter how much acid or base was added. The slope of the line is small.
D. The buffer was not able to maintain a constant pH. The line has a very steep slope in two regions - one in the acidic region and one in the basic region.
2. Which of the following is the strongest base?
A. all three are of equal strength
B. ammonia
C. diluted bleach
D. acetone
Experiment 2
3. Which of the four solutions tested is the best buffer against changes in pH caused by the addition of acid or base?
Experiment 3
4. Which of the following is true regarding how weak acid-conjugate base buffers neutralize acids and bases?
A. The conjugate base neutralizes bases. The weak acid neutralizes acids.
B. The weak acid in the buffer is attracted to strong acids and surrounds them, neutralizing them. Conjugate bases act in a similar fashion on strong bases.
C. strong acid and weak acid bond, forming a compound that is not acidic. A similar reaction occurs when a strong base is added to a buffer.
D. The weak acid in the buffer neutralizes bases. The conjugate base neutralizes acids.
5. Suppose you had a buffer containing 0.5 moles of sodium monobasic phosphate and 0.5 moles of sodium dibasic phosphate. How many moles of hydrochloric acid would this phosphate buffer be able to accept before the pH of the solution began to change drastically?
6. What is the biological significance of buffers?
A. All of the answer choices are examples of the biological significance of buffers.
B. Buffers prevent stomach acids from escaping the stomach and digesting other tissues in the body.
C. Buffers prevent acidic or basic foods from altering the pH of the digestive system.
D. Buffers prevent even the slightest changes in pH that can inhibit important biological molecules such as enzymes.
7. 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
8. Explain why phosphate buffer needs both Na2HPO4 and NaH2PO4 in order to resist changes in pH.
9. Which chemical provides the conjugate base in the buffer containing NaH2PO4 and Na2HPO4?
10. 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?
11. Predict what might happen if you made up phosphate buffer with only half as much NaH2PO4 compared to Na2HPO4.
A. Initially, the buffer was not able to stabilize the pH and the graph changed dramatically. However, after an initial change, the buffer was able to draw the pH closer to neutral number.
B. The buffer was able to keep pH changes small for a large number of drops, but past a critical point the pH changed dramatically.
C. The buffer kept the change in pH moderate no matter how much acid or base was added. The slope of the line is small.
D. The buffer was not able to maintain a constant pH. The line has a very steep slope in two regions - one in the acidic region and one in the basic region.
Explanation / Answer
1. B : The buffer was able to keep pH changes small for a large number of drops, but past a critical point the pH changed dramatically.
2.A : Ammonia is the strongest base
4.D. The weak acid in the buffer neutralizes bases. The conjugate base neutralizes acids.
5. when you add acid , di basic phosphate will be converted to monobasic phosphate. When all the dibasic phosphate is converted to mono basic phosphate , the solution will loose its buffer ability. So, moles of HCl that shuld be added = 0.5 moles
6.D. Buffers prevent even the slightest changes in pH that can inhibit important biological molecules such as enzymes.