0

Will the Henderson–Hasselbalch equation be able to calculate the pH when you have an acidic solution and you add a basic solution? Will that be possible or can you only use it for adding an acid to an acidic buffer to calculate the pH, or only a base to a basic buffer? If it can, how is the pH calculated, if you add 0.025 mol of solid KOH to 1 L of an acidic acid buffer solution of pH 3.97.

orthocresol
  • 71,033
  • 11
  • 239
  • 410
Asker123
  • 3,050
  • 7
  • 22
  • 48
  • 1
    In case of very dilute buffer solutions, you shold try the Brönsted–Charlot equation: https://en.wikipedia.org/wiki/Charlot_equation – Gabor Nagy Aug 22 '17 at 08:34
  • Asker123, when you originally asked this question (before people edited it), did you mean that we start with pure pH 3.97 acetic acid solution and add base in an attempt to create a buffer, or did you mean that we start with pH 3.97 buffer? – DavePhD Aug 24 '17 at 16:35

1 Answers1

4

If the initial solution is just acetic acid and water, and the pH is 3.97, this means that it is a very dilute solution, less than 0.001M acetic acid. See http://depts.washington.edu/chem/facilserv/lecturedemo/pHofAceticAcid-UWDept.ofChemistry.html

and Does the number of H+ ions in solution go up on dilution of a weak acid?

Therefore, of the 0.025 moles of OH-, almost all (more than .024 moles) will remain unreacted.

pH = 14 - pOH

pH = 14 + log(0.025) = 12.4

In a different situation, where you added less strong base than weak acid, the Henderson-Hasselbach equation would be helpful, but it is not helpful in this example.

Community
  • 1
DavePhD
  • 40,570
  • 2
  • 85
  • 181
  • You are adding OH- to the solution – Asker123 Feb 28 '15 at 17:21
  • yes, because acetic acid concentration will be less than 0.001M, almost all the OH- will stay OH- – DavePhD Feb 28 '15 at 17:23
  • Alright then but can you find the pH of a weak acid buffer when you add OH- with an equation?\ – Asker123 Feb 28 '15 at 18:24
  • Yes, use the Henderson Hasselbach and don't add more OH- than weak acid – DavePhD Feb 28 '15 at 18:30
  • The question makes no comment about the concentration of the buffer. It should however be noted, that pure acetic acid is no buffer, and your answer only accounts for this case. – Martin - マーチン Aug 22 '17 at 12:50
  • @Martin-マーチン on this SE, you always have to dig back through the overenthusiastic editors' efforts to find what the real question was. There was no "buffer" in the original title. The original body did not say "acidic buffer". OP wants to know "the pH when you have an acidic solution and you add a basic solution" and said "Has pH of 3.97 and is 1L. Acetic acid to be precise." So, I interpret pH 3.97 "acetic acid to be precise" as pure acetic acid solution. – DavePhD Aug 22 '17 at 13:28
  • @DavePhD it said buffer in the question, that's why I edited it this way, and I was not overenthusiastic while editing. I would have closed/deleted the question on sight and it only came to my attention because of a flag. I'm sorry this caused inconveniences. – Martin - マーチン Aug 22 '17 at 13:40
  • @Martin-マーチン ok, please delete my answer. Moderator approval needed to delete accepted answer. – DavePhD Aug 22 '17 at 13:52
  • @Martin-マーチン I'm not sure why this answer would require deletion, but the question would not. – hBy2Py Aug 24 '17 at 16:18
  • @Martin-マーチン I'll ask OP to tell us if he meant pure pH 3.97 acetic acid solution, or something else. I thought his intent was: start with pure pH 3.97 acetic acid solution and add base, in an attempt to a create buffer. If OP indicates pH 3.97 buffer of unspecified composition/concentration was intended, then my answer should be deleted. Otherwise the question should be re-edited. – DavePhD Aug 24 '17 at 16:33
  • I actually like your answer, and find it helpful. The question is a bit vague on details, but with your answer I would not want to delete it. My initial point was simply that I thought the question was unclear. Given your very first statement there is no misinterpretation possible. In doubt, I'd just leave it as is. – Martin - マーチン Aug 24 '17 at 16:46
  • I agree. Even though the question is highly non-ideal (confused/poorly grounded in more than one way, at minimum), I think this answer was really well done in response to it. Firmly in favor of keeping it here. – hBy2Py Aug 26 '17 at 03:41