Weak Acid-Base vs Buffer problems

[H2Otoo]

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How do you recognize which one is weak acid-base equilibria and which one is a buffer problems when they do not tell you that it's a buffer problem? What is the difference?

 

[tturchi51]

A buffer solution is a mix of a weak acid with its conj base (or vice versa) in as close to a 1:1 ratio as possible

3 ways to make buffers

1. 1 equivalent weak acid/ 1 equivalent conj base

2. 1 eq weak acid/ (1/2) eq strong base

3. 1 eq weak base/ (1/2) eq strong acid


Examplesx

.1 M HBr + .1 M KBr (Not a buffer b/c its a Strong acid and conj base)

.1 M HClO4 + .1 M KOH (Not a buffer bc Strong acid and strong base)

.1 M HClO2 + .1 M NaClO2 (buffer! Weak acid and conj base in a 1:1 ratio)


o Acids-CH3COOH, CH3COONa, HF, KF

o Bases- NH3, N H4Cl, C H3NH2, C H3NH3+Cl-



Also if you are having trouble with chem or orgo use Chad's vids and quizzes from coursesaver.

 

[Altius Premier Tutor]

That's kind of like asking, how do you tell when it is "two people pulling in opposite directions on either end of the same rope" or a tug-of-war? A buffer IS a weak acid-base equilibrium. The reason a buffer buffers (read: resists changes in pH even when you drop in mega strong acids or bases) is because it is an equilibrium established between that weak acid and its conjugate base or the weak base and its conjugate acid. The added acid or base disrupts the system and according to Le Chatelier's it will resolve back toward equilibrium as quickly as possible (meaning no major pH change).

So, to answer your question, I would say don't try to see them as different, when you see "weak" combined with acid/base...and definitely if you see that with the word "equilibrium" think BUFFER! The other clue you'll see on AAMC MCAT questions is 1:1 ratios. If you see 0.1 M of this and 0.1 M of that....most the time this and that are going to be a weak acid/base pair buffering. Also look for that "only 1 hydrogen" difference on anything you know isn't strong--that's another clue its a buffer (e.g., CH3COOH/CH3COO-, NH3/NH4+, H2CO3/HCO3-...you get the idea).

 

[NextStepTutor_3]

@tturchi51 already mentioned this, but it's worth repeating: a buffer CAN be made by mixing a weak acid (or base) and a smaller number of moles of strong base (or acid). This is a really common way that the MCAT disguises buffer questions. For example, consider a solution where 0.5 mol NaOH has been added to 1 mol HF. Often, people eliminate answers like this right away because they see the strong base, and know that buffers must contain only a weak acid / base and its own conjugate. What they fail to realize is that this mixture does produce a buffer - in fact, it produces one that is perfectly buffered at the pKa of HF. NaOH deprotonates exactly 0.5 mol of the original HF, leaving us with 0.5 mol F- and 0.5 mol HF remaining. Note that to make a buffer, we don't need the strong acid or base to be exactly half the quantity of the weak acid or base, but we certainly need it to be present in a smaller amount.

There are tons of awesome practice passages out there that test this exact topic, I know off the top of my head that BR has some great ones. I advise doing a LOT of acid-base practice - it's always been very high-yield.