(1a) Calculate the pH of a 0.20 M HNO/0.20 M NaNO, buffer. What is the pH of the

Question

(1a) Calculate the pH of a 0.20 M HNO/0.20 M NaNO, buffer. What is the pH of the solution after the addition of1 1. (60 pts) (more) Acids and Bases 0 (1b) For a particular experiment, Candy Chemist, our favorite acid-base buffer savant, must prepare a buffer with a pH 8.50. In the laboratory, she finds 1.00 M acetic acid (Ka 1.76x10) and 1.00 M ammonia (Kb1.76x10). Which reagent should be used to prepare the buffer? In addition to the above question, your assignment is to calculate the mole ratio of acid to conjugate base (or base to conjugate acid), which can be used to determine the best reagent to prepare the buffer. Is the buffer better for additions of acid or base? Explain.

Explanation / Answer

(1a) pH calculation

Using hendersen-Hasselbalck equation,

pH = pKa + log(NaNO2/HNO2)

     = -log(4 x 10^-4) + log(0.2/0.2)

     = 3.40

After NaOH = 0.1 M x 10 ml = 1 mmol added

pH = 3.40 + log[(0.2 M x 90 ml + 1 mmol)/(0.2 M x 90 ml - 1 mmol)]

      = 3.45

(1b) The reagent whose pKa is closest to pH needed for the buffer would be used in this case.

pKa of acetic acid = -log(1.76 x 10^-5) = 4.75

pKa of NH3 = -log(1 x 10^-14/1.76 x 10^-5) = 9.24

So we would use NH3/NH4+ combination to prepare the base.

Using hendersen-Hasselbalck equation,

8.50 = 9.24 + log(NH3/NH4+)

so,

moles ratio of (NH3/NH4+) = 0.181

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