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.
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1$\begingroup$ In case of very dilute buffer solutions, you shold try the Brönsted–Charlot equation: en.wikipedia.org/wiki/Charlot_equation $\endgroup$– Gabor NagyAug 22, 2017 at 8:34
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$\begingroup$ 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? $\endgroup$– DavePhDAug 24, 2017 at 16:35
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1 Answer
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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.
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$\begingroup$ yes, because acetic acid concentration will be less than 0.001M, almost all the OH- will stay OH- $\endgroup$– DavePhDFeb 28, 2015 at 17:23
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$\begingroup$ Alright then but can you find the pH of a weak acid buffer when you add OH- with an equation?\ $\endgroup$– Asker123Feb 28, 2015 at 18:24
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$\begingroup$ Yes, use the Henderson Hasselbach and don't add more OH- than weak acid $\endgroup$– DavePhDFeb 28, 2015 at 18:30
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$\begingroup$ 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. $\endgroup$– Martin - マーチン ♦Aug 22, 2017 at 12:50