Chemistry 122 Go Exam II Page 7 of 9 Which one of the following would be the bes

Question

Chemistry 122 Go Exam II Page 7 of 9 Which one of the following would be the best buffer a pH 3.20 solution? for Spring 2015 a. A mixture with equal moles of HCN and NaCN.pH spNa q.zl b. A mixture with equal moles of HCl and dud olution NaCN. Weg c. A mixture with equal moles of HF and NaO N solvtill d. A mixture with twice as many moles of HF as NaF. ft 3.nlt l0,9) e2 e. A mixture with equal moles of HF and NaF. p Pkd IT Problem Solving Section: Each question is worth the points listed in the parenthesis. Partial credit will be given. 11. For the following questions, consider the reaction below Mdn

Explanation / Answer

Using Henderson-Hasselbalch equation:

pH = pka+log ([A-]/[HA])

So now pH = 3.2

3.2 = pKa + log ([A-]/[HA])

So for a buffer to suitable to maintain the pH of 3.2, it's pKa must be near to pH and the ratio of concentration of acid and its conjugate base (ratio of moles if volume is constant) must be equal to 1. To make up a buffer solution of a certain pH, it is suitable to use a weak acid. HF is a relatively weak acid and also out of all the options mentioned, it's mixture of equal moles of HF and NaF has pKa very close to 3.2 and also the number of moles of HF and NaF are same,i.e the ratio of concentration of acid and its conjugate base. So this would be a preferrable buffer solutions.So with pKa almost equal to 3.2 and ratio of moles of acid and conjugate base equal to 1, it will help in maintaining the pH of 3.2.

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