[nr101801]

Hi,
I am a first year chemistry student and plan on majoring in chemistry as my career. But I am stumped on the topic of buffers when it comes to acids and bases.

You run an experiment to determine the buffer capacity of a 0.50 M aceti acid/ acetate buffer. You use 22.87 mL of 0.0878 M KOH to titrate 10.00 mL of the buffer solution and effect a change of 1.02 pH units. What is the buffer capacity.

I calculated
0.002008 moles/((1.02pH)x(.01L)=.19686 <<<<<<<Is this the correct answer<<<<<<

Also
1. What relationship does buffer capacity versus buffer concentration have?
2. Can a buffer be formed from HNO3/NO3-? Why or why not?
3. What could you do to the concentration of the constituents of your buffer solutions to optimize them for titration with a strong acid?

I am emailing from Anchorage, Alaska and need guidance. Please email me if you can help.
Thank You:)

[Perito]

You run an experiment to determine the buffer capacity of a 0.50 M acetic acid/ acetate buffer.

You use 22.87 mL of 0.0878 M KOH to titrate 10.00 mL of the buffer solution and effect a change of 1.02 pH units. What is the buffer capacity.

I calculated
0.002008 moles/((1.02pH)x(.01L)=.19686 <<<<<<<Is this the correct answer<<<<<<

Also
1. What relationship does buffer capacity versus buffer concentration have?
2. Can a buffer be formed from HNO3/NO3-? Why or why not?
3. What could you do to the concentration of the constituents of your buffer solutions to optimize them for titration with a strong acid?

22.87 mL x 0.0878 mmoles/mL = 2.008 mmoles of KOH (0.002008 moles). That is the buffer capacity. It's the capacity of this buffer to absorb hydroxide. The pH change is not affected linearly by base consumption. The pH change simply indicates that you are close to the end point (end of capacity of the buffer). Basically, that means that all of the buffer acid has been converted into the salt in this case.

1. The buffer concentration affects the buffer capacity. The more buffer that is present the more acid or base that can be absorbed. The pH of a buffer solution is largely independent of the concentration of the buffer. It is slightly dependent on the acid/salt or base/salt molar ratios.

2. No. Strong acids cannot be used as buffers. The buffer phenomenon requires a weak acid (or weak base)/salt combination. The salt reacts with acid (to form the unionized weak acid), the acid releases protons (hydrogen ions) that react with bases.

3. "What could you do to the concentration of the constituents of your buffer solutions to optimize them for titration with a strong acid?"

I'm not sure I understand that question. Are you creating the buffer by adding a salt (example: sodium acetate) to a solution and titrating half of it with acid so that you have an equimolar amount of acetate and acetic acid? You can build buffer solutions in this way or you can simply mix acetic acid and sodium (or some other cation) acetate.

Titration is not something that is often done with buffer solutions -- unless you absolutely must know the buffer capacity. If you wanted to optimize for titration, I guess you could make the concentration such that it would be easy to titrate with one burette of a particular concentration of acid, but that might defeat the idea of making a high buffer concentration.

By the way, there is an other consideration when dealing with buffers. That is the concept of "ionic strength" or "ionic concentration", the number of ions in a given amount of solution. Especially in biological systems, if the ionic strength is too great, it can kill the organism. There can be other limitations to the ionic strength in other systems, also. Furthermore, you can't create a buffer with an huge capacity -- once you reach saturation, you've surpassed the useful limit on concentration.