Suppose you need to prepare 250.0 mL of a pH=3.60 buffer that has a total buffer concentration of formic acid + formate of 0.030 M. How many moles of formic acid will you need? The pKa of formic acid is 3.75.

The number of moles of formic acid needed is 4.5x10⁻³ moles.

Explanation:

We can find the moles of formic acid using the Henderson-Hasselbalch equation:

We know:

pH = 3.60

pKa = 3.75                    

By solving the above equation for [CHOO⁻]/[CHOOH] we have:

 (1)

Now, we have:

  (2)

By entering equation (1) into (2) we have:

Hence, the concentration of formate is:

Finally, the number of moles of formic acid is:

Therefore, 4.5x10⁻³ moles of formic acid are needed.

I hope it helps you!  

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