Calculating Acid:Buffer Ratio using H-H Equation

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Given Question: Which acid would you combine with its Na (sodium) sat to create a buffer with a pH of 7.35. What ratio of a buffer to acid is needed? There are 3 potential buffers provided to choose from.

1. First, we choose which buffer is the best acid to choose to obtain the desired pH. Our strategy is to choose whichever acid provided has the closest pKa value to the desired pH. Our pH needed is 7.35, and our closest pKa value is 7.54- so our buffer is HClO. This reflects the concept that buffers can accept or lose many protons and still be able to maintain a steady pH level.

2. Now that we have our acid identified, we can use the Henderson-Hasselbach to find the ratio of buffer acid to base. The values we already have can be plugged in.

3. Plug in  the desired pH in the H/H equation and the provided pKa value (respectively 7.35 & 7.54) set this equation equal to the [A-]/[HA] value. Since there can’t be a negative value for this equation, the difference between the pH and pKa will be the absolute value.

4. Now that we have our 0.19 value equal to the log of [A-]/[HA], you can now find the ratio value. Remember, in order to calculate out our log expression, we can raise ten to both sides of the equation. This will result in our simplified buffer acid:base fraction and hopefully the right value for it!

When going over this particular question, I got 1.54 as my final value. And seeing as these notes are from a few years ago, I can’t verify the right answer. The process, however, is right. I hope. So please send feedback on what you got, so we know which answer is correct! Or if either of them are correct.

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