Best optimal buffer, highest buffering capacity against naoh

[wombaticus]

Homework Statement

Group#1: Buffer pH = 4.00 Group#2: Buffer pH = 4.35 Group#3: Buffer pH = 4.70 Group#4: Buffer pH = 5.00 Group#5: Buffer pH = 5.30 Group#6: Buffer pH = 5.60
1. Explain which group should havethe BEST OPTIMAL BUFFER
(see choices above).
2. Explain which group has a buffer that has the HIGHEST BUFFERING CAPACITY AGAINST NaOH.
3. Explain which group has a buffer that has the HIGHEST BUFFERING CAPACITY AGAINST HCl.
4. Is the best optimal buffer the same as the buffer that has the HIGHEST buffer capacity againstNaOH? Explain your reasoning.
5. Is the best optimal buffer the same as the buffer that has the
HIGHEST buffer capacity against HCl? Explain your reasoning.

Homework Equations

There aren't any equations needed to do these problems. You can answer all of them knowing the literature value of acetic acid, whose pka is 4.74

The Attempt at a Solution

1. group 3 because optimal buffer is one whose ph = pka.
2. ??
3. ??
4 and 5. I know the answer is no but I don't know why

 

[VanessaN]

Buffers are compounds that resist changes in pH. Buffering capacity= Moles of acid or base added in buffers solution / change in solution's pH. The greater the buffering capacity, the greater the quantity of acid or base which must be incorporated with a material to alter the pH.
I would think that the highest buffering capacity against NaOH would be the highest pH, the highest buffering capacity against HCl would be the lowest pH. pka of HCl is -4, pkb of NaOH is 0.2

http://chemcollective.org/activities/tutorials/buffers/buffers5
http://www.chembuddy.com/?left=BATE&right=dissociation_constants

 

[Borek]

Explain which group should havethe BEST OPTIMAL BUFFER

Bets optimal for what? There is no one buffer that is "best" in general, buffers are good for specific tasks.

Question doesn't say what weak acid the buffer is based on. You can be right about acetic acid.

Then, there is a problem with the definition of buffering capacity. What definition were you given and what definition are you expected to use? There are several competing definitions and to some extent the answer can depend on which one you use. See the discussion at http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

In general capacity is the highest for pH=pKa (regardless of whether you add acid or base).

 

[epenguin]

Well you shouldn't need to be helpless in answering 2 & 3. You had no manual or text? No ability to derive yourself the equation for β from its definition?

If not you now have the links from Borek - which will show you, I hope, that you could have done it yourself (unless you have a problem with very elementary algebra and differentiation) and hopefully will be able to do so without problem in future - because having that capacity is frankly less trouble than trying to remember formulae. You also need to back this up with some qualitative reasoning and intuitive understanding about why buffering capacity is greatest in the regions where it is.

With respect to the links and some others I have seen may I say that in the situation of the problem and most that you will be asked about, you can expect practically C_buf to be so much higher than [H^+]] or [OH^-] that you can ignore two of the terms and so simplify your equations? You may find in some texts this has been done, with or without mentioning it explicitly.

You will not get the best benefit from this site unless you submit your answers and see the problem through to the end.

 

[wombaticus]

the weak acid is acetic acid. the definition we are using is maximum amount of either strong acid or strong base that can be added before a significant change in the pH will occur

 

[epenguin]

the weak acid is lactic acid. the definition we are using is maximum amount of either strong acid or strong base that can be added before a significant change in the pH will occur

j

That is not an attempt at an answer, it is repeating the question (almost - because we have been told twice it's acetic and once it is lactic, so what is it and what pK are you using?) Your answer 1 is almost right btw for acetic - the nearer pH is to the pK_a the higher is the buffering capacity. (Which it is possible to deduce from equations already linked to).

 

[wombaticus]

klkkk

 

[Borek]

significant change in the pH

Define "significant". 0.1? 0.5? 1? More than 2?