Question on how to prepare a buffer solution


by KyoPhan
Tags: buffer
KyoPhan
KyoPhan is offline
#1
Jan10-09, 09:15 PM
P: 13
I don't know if its just me, but I think the wording of the question is weird and I'm not really understanding what it is saying. I have stared at this problem for many hours and have even went to the teacher to try to clarify. The teacher provide us with the actual work on how to calculate it, but I do not understand the steps she has taken.


1. The problem statement, all variables and given/known data

Here's the question:
Make 200 mL of 0.1M Na-acetate buffer, pH 5.1, starting with 5.0 M acetic acid and 1.0M NaOH.

2. Relevant equations
pKa of acetic acid = 4.76
pH = pKa + log([Ac-]/[HAc]


3. The attempt at a solution

I'm really struggling on what the question is saying, particularly with the part where it says "Make 200mL of 0.1M NAa-acetate buffer".

I cannot really start the problem off because I do not understand what it wants. If someone can maybe explain what it is saying to start me off, I think that would be really helpful to me. Thanks a lot.
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epenguin
epenguin is offline
#2
Jan10-09, 11:18 PM
HW Helper
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P: 1,932
Maybe first problem is the ambiguous sounding "0.1M Na-acetate buffer"? It can't in general be 0.1 M both in Na and in acetate. When you say xM buffer, it is meant xM in the buffering substance, the one that associates-dissociates.

So this is 0.1M in total acetate in both forms.
[HAc] + [Ac-] = 0.1

So the amount of Acetic acid you need to use is the amount that makes 200 ml of 0.1 M.

That's supposed to be the easy part.

Now you give an equation in which you know two of the quantities.

Problem is the other two.

Now what else is [Ac-] equal to?

What other constraints have you got, which will enable you to work out everything?

As yours is practically the most common question that comes up I wrote out previously what you have to do in all cases:
http://www.physicsforums.com/showpos...73&postcount=3

but here is the main bit:

For all questions you are going to meet in this area and which do cause headscatchings to students just remember all proceed from just 3 principles, at a stretch 4, always the same

1. Conservation of mass, i.e. of the total concentration of the amount of any one species summed over all its forms;

2. Electroneutrality - equality between the sum of all the + charge (per litre let's say) OT1H and all the - charge OTOH;

3. The equilibrium laws that apply.

All 3 embodied in equations that correspond to the case.


4. In some problems you may need also to understand what approximations you can use (i.e. what concentrations in your equations are going to be negligible compared to others and that you can ignore to shortcut or simplify the maths.) You will also need understanding of what logs are, especially logs to base 10.

Just write out together all the equations that correspond to 1, 2 and 3 above and see what you can do with them now. At some point you also put in numbers that you are given - add them to the same equations like my equation above which is a conservation of mass equation.
KyoPhan
KyoPhan is offline
#3
Jan11-09, 08:44 PM
P: 13
Ahh, after staring at it for a while I think I finally understand now. It was just that 0.1M buffer part that was confusing me. Thanks a lot


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