Buffers weak acids conjugate base

Question

Choose any weak acid from ka table list that will buffer at the following pH values. Choose whose ka is closest to the requested pH value. Also determine the ratio of weak acid to conjugate base( HA/A^-) of the resulting buffer solution.

1.) pH = 5.00

2.) pH= 4.67

3.) pH= 1.75

4.) pH= 3.65

5.) pH= 9.45

( I used a ka list online to help myself figure out the ka of any weak acids.)

Answer 1

I will do the first one for you, then you should be able to do the others yourself.

The Henderson-Hasselbalch equation is pH = pKa + log conjugate base/ conjugate acid.

We need to know the pH desired for the buffer which was given as 5.00. A table of pKa of weak acids was consulted on line and I found acetic acid at a pKa of 4.76 which is reasonably close to the desired pH. Now, all we have to do is plug these numbers into the equation and solve for log conjugate base/ conjugate acid.

So, 5.00 = 4.76 + log Ac^-/ HAc

log Ac^-/ HAc = 5.00 - 4.76 = 0.24

Ac^- / HAc = antilog of 0.24 which is 1.74, but the question asks for the ration of acid to conjugate base, so we need to invert the result to get 1/1.74 = 0.575.

The remaining problems can be done similarly.