If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, determine the [HCHO2] if the [NaCHO2] is 0.735 M.

Answer 1

The system described represents a buffer system containing a weak acid (HCHO₂) and the conjugate base (NaCHO₂). For this calculation, we can use the Henderson Hasselbalch equation:

pH = pKa + log [conj.base] / [acid]

We are given the pH, pKa and [NaCHO₂]. Solve for [HCHO₂]

3.89 = 3.74 + log (0.735/x)

0.15 = log 0.735/x

1.41 = 0.735/x

x = 0.521 M = [HCHO₂]