Determining Minimum NaOH Amount for 0.4M Acetic Acid Buffer Capacity

[apchemstudent]

What is the smallest amount of solid NaOH that will exceed the buffer capacity 500mL of a buffer that is 0.4M in acetic acid and 0.15M in NaOAc?

a)5.69 b)5.16 c) 4.74 d) 4.31 e)3.00 grams

Since this is a strong base, we only need to deal with the 0.4M of acetic acid.

so 0.4M *.5L = 0.2moles of NaOH

that's 8 grams. What am I doing wrong here?

 

[siddharth]

I think you should read up on what a buffer is.
http://en.wikipedia.org/wiki/Buffer_solution"

Buffer solutions are solutions which resist change in hydronium ion concentration (and consequent pH) upon addition of small amounts of acid or base, or upon dilution

So, can you see why what you did is wrong?

 

[apchemstudent]

I think you should read up on what a buffer is.
http://en.wikipedia.org/wiki/Buffer_solution"



So, can you see why what you did is wrong?

oops... i think i got the answer... thanks

 

[Borek]

Seems there are different definitions of buffer capacity in use. I was taught buffer capacity is defined as the ratio of the amount of strong base or strong acid added to the change in pH - thus you can't say when it will run out - as it is value that you can calculate for a point (as first derivative, dn/dpH, where n - moles).

 

[Borek]

Buffer capacity definition and some discussion.

 

[Geir]

Does anyone know what the answer to that question is? I tried to figure it out but I'm getting enough of a pH change for any of the amount of solid.

 

[Borek]

First of all - what pH change do you expect?