1) Based on the pH of solution A, what is the pKa of acetic acid? Over what rang

Question

1) Based on the pH of solution A, what is the pKa of acetic acid? Over what range of pH values would an acetic acid/acetate buffer be effective?

2) Based on the pH of solution B, what is the pKa of H2PO4?? Over what range of pH values would a dihydrogen phosphate/hydrogen phosphate buffer be effective?

3) Why do solutions A and B have different pH’s?

4) In Experiment Step I, how do the pH’s of the “2” beakers (each with 3 drops of 1 M HCl added) compare with each other and with the “1” beakers?

5) In Experiment Step I, how do the pH’s of the “3” beakers (each with 3 drops of 1 M NaOH added) compare with each other and with the “1” beakers?

6) How did the behavior of the solutions in experiment step II compare with their behavior in step I? Why was there a difference?

7) Most living cells function in a pH range around 7.0. Which solution used in this experiment (distilled water, solution A, or solution B) would you choose if you wanted a solution to maintain a neutral simulated cell system? Explain your choice.

8) If you start with 30.0mL of solution A and you add 2.0mL of 0.50 M HCl, calculate the pH of the final solution.

9) If you start with 80.0mL of solution B and you add 10.0mL of 0.60 M NaOH, calculate the pH of the final solution.

Explanation / Answer

1) For sodium acetate (A = CH3COO-), pKa of acetic acid is 4.76.

At pH range 3.74 to 5.74, acetate buffer is effective.

2) For B = HPO42-, H2PO4- has pKa of 7.2.

At pH range 6.0 to 8.0, phosphate buffer is effective.

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