Which buffer system is the best choice to create a buffer with pH pH = 7.20? (A) HC, H, O, KC, H, O, (B) HCIO, / KCIO, (C) HCIO / KCIO (D) NH / NH, Cl Answer OA B D Submit
Answers
Answer:
Part A: Which of the following buffer systems would be the best choice to create a buffer of pH = 7.20?
(a) CH3COOH & CH3COOK
(b) HClO2 & KClO2
(c) NH3 & NH4Cl
(d) HClO & KClO
Part B: For the best system, calculate the ratio of masses of the buffer components required to make the buffer.
Solution to A:
1) Examine the pKa of each acid:
(a) Ka = 1.77 x 10¯5; pKa = 4.752
(b) Ka = 1.1 x 10¯2; pKa = 1.96
(c) Ka = 5.65 x 10¯10; pKa = 9.248
(d) Ka = 2.9 x 10¯8; pKa = 7.54
2) Write the Henderson-Hasselbalch equation:
pH = pKa + log [base / acid]
3) Set the base/acid ratio to 1/1 and the result is:
pH = pKa
4) Look for the pKa value nearest to a pH of 7.20 when the base/acid ratio is 1/1:
the best choice for a pH = 7.20 buffer is answer choice d, the HClO & KClO buffer
5) Why?
The buffer with a pKa closest to 7.20 would be the best choice because it has the least adjusting to do in the base/acid ratio in order to reach a buffer of pH = 7.20. I will return to this below.
Solution to B:
1) Let's write the Henderson-Hasselbalch equation again:
pH = pKa + log [base / acid]
2) We know the desired pH of the buffer, so let's put it in:
7.20 = pKa + log [base / acid]
3) Let's get the pKa of HClO:
I will use a pKa equal to 7.5376 (I'll keep a couple extra digits for the moment.)
4) Put it in place:
7.20 = 7.5376 + log [base / acid]
log [base / acid] = −0.3376
[base / acid] = 0.45962 (still keeping some extra digits)
5) The above ratio is a ratio of moles, but we want a ratio of weight. What we will do is use the molar masses of KClO and HClO and change the mole ratio to a weight ratio.
numerator (KClO) = (0.45962 mol) (90.55 g/mol) = 41.6186 g
denominator (HClO) = (1 mol) (52.4599 g/mol) = 52.46 g
The weight ratio is 41.6186 g / 52.46 g = 0.79 (to two sig figs)
Explanation:
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