A buffer solution of formic acid (Ka = 1.78 x 10-4) has 3500 times more formate ions than formic acid molecules. What is the pH of this buffer solution?

Answer 1

To find the pH of the buffer solution, we can use the Henderson-Hasselbalch equation, which relates the pH of a solution to the pKa of the acid and the concentration ratio of the acid and its conjugate base:

pH = pKa + log ([A-] / [HA])

where
pH is the pH of the solution,
pKa is the -log(Ka) of the acid
[A-] is the concentration of the conjugate base (formate ions)
and [HA] is the concentration of the acid (formic acid molecules).

First, we need to find the pKa from the given Ka value:
pKa = -log(Ka) = -log(1.78 x 10^-4) ≈ 3.75

Next, we have the concentration ratio of formate ions to formic acid molecules as 3500. Since [A-] = 3500 * [HA], we can write this ratio as follows:

pH = pKa + log (3500 * [HA] / [HA])

Now, we can cancel out the [HA] from the numerator and denominator, so we have:

pH = 3.75 + log(3500)

Calculating the log of 3500 gives:

log(3500) ≈ 3.54

Finally, add this value to the pKa:

pH = 3.75 + 3.54 ≈ 7.29

So, the pH of this buffer solution is approximately 7.29.