Formic acid is the active agent in an ant bite. What is the ratio of base/acid for formic acid (pKa = 3.9) in the blood stream at pH 7.4?

Question

Quizplus · Accepted Answer

The ratio of base/acid is determined by the Henderson-Hasselbalch equation: 
pH = pKa + log([base]/[acid])
At pH 7.4 and pKa 3.9 we can solve for the ratio:
7.4 = 3.9 + log([base]/[acid])
3.5 = log([base]/[acid])
Antilog of 3.5 = [base]/[acid]
Antilog of 3.5 is approximately 3.16 × 10^1.3 (use logarithm rules, i.e. antilog of 3.5 = 10^(3.5) = 10^(3 + 0.5) = 10^3 × 10^0.5 = 1000 × 3.16 = 3162)
Therefore, [base]/[acid] = 3.16 × 10^1.3, which can be expressed in scientific notation as 3.16 × 10^(1.3-4) or as 0.00316. 
This is accurately represented in answer choice B, which is 3.16 × 10^1.3 : 1 or approximately 316 : 1.

Formic Acid Is the Active Agent in an Ant Bite