Chads: Question about buffers?

[HdK]

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I wrote in my notes from Chad's that there are 3 ways to make buffers..and two of the examples say weak acid+conjugate base, weak base +strong acid.. I'm confused now and Ive expired my subscription =(..I'm confused cuz I've looked online and they say to make a buffer you must use weak acid + conjugate base or weak base + conjugate acid, so why did chad give an example of weak base and strong acid? He specificially said NaF + HCl.

Can someone clarify if anything what EXACTLY can make a buffer? Thanks so much.

 

[SafetyDog]

You have to have double the amount of the weak acid than the strong base. What happens is the that the base will react with the weak acid, yielding a weak base> However, you still have some weak acid left over because you had double the amount of weak acid than strong base. This leaves you with a weak acid + weak base.

That's what I can remember. It's something along those lines.

 

[hope_to_match]

Chad gave three examples to make a buffer (a buffer resists changes in ph) so...

a) you can have weak acid and it's conjugate base i.e. HF+NaF in 1:1 ratio
b)strong acid and weak base i.e. HCl +NaF in 1:2 ratio (you need twice the amount of base since your acid is strong)
c) strong base and weak acid i.e. NaOH+HF in 1:2 ratio (again you need twice the amount of the weak stuff to make up for the other strong)

I hope this helps!

 

[HdK]

Thanks to both of you. very very helpful.

 

[Jay20155555]

Chad gave three examples to make a buffer (a buffer resists changes in ph) so...

a) you can have weak acid and it's conjugate base i.e. HF+NaF in 1:1 ratio
b)strong acid and weak base i.e. HCl +NaF in 1:2 ratio (you need twice the amount of base since your acid is strong)
c) strong base and weak acid i.e. NaOH+HF in 1:2 ratio (again you need twice the amount of the weak stuff to make up for the other strong)

I hope this helps!

isnt a and c the same thing? since you have a weak acid wont the conj base be strong?

 

[Incis0r]

isnt a and c the same thing? since you have a weak acid wont the conj base be strong?

This is where many chemistry curricula mislead students.
The answer is no.

Strong acids have NEGLIGIBLE conjugate base strength.
Weak acids have WEAK conjugate base strength.
Negligible acids have STRONG conjugate base strength

Look at this image:

Example:
HCl- strong acid right? Well Cl- is a negligible base. Extremely stable on its own. Non-reactive.
HC2H3O2- acetic acid. Weak acid right? Well, C2H3O2- is a weak conjugate base.

See what I'm saying?

A and C are different as follows:
a) weak acid + conjugate base: HF is the weak acid. The conjugate base is in salt form, so NaF. So, from the start, in solution you have [HF] and [F-]. You don't "make" anything. It's readymade.
c) strong base and weak acid: NaOH and HF. HF is the weak base. You add 2 equivalents of the HF.
HF + NaOH -> H2O + F-
I 2 1 0 0
C -1 -1 +1 +1
E 1 0 1 1

So you PRODUCE the conjugate base (F-) from the weak acid (HF) by means of a strong base (NaOH). Make sense? This differs from A in that, in A, the [F-] is already in solution thanks to the salt NaF. In this case, you PRODUCE the F-.