Question

A solution of H₂CO₃ with pKₐ 1=6.35 and pKₐ 2=10.33, has a concentration of 0.740M (Note that a pKₐ value is provided. pKₐ = -logKₐ) Determine the pH of this solution:

Answer 1

To calculate the pH of a 0.033 M H2CO3 solution, use the hydrogen ion concentration from the first ionization step and apply the pH formula, resulting in a pH of 3.92. The more significant first ionization step dominates, thus simplifying the calculation.

Step-by-step explanation:

To determine the pH of a solution of H2CO3 with known pKa1 and pKa2 values, we need to consider both ionization steps. Given [H2CO3] = 0.033 M and [HCO3­] = [H3O+] = 1.2 × 10—4 M from the first ionization, we can apply the Henderson-Hasselbalch equation for the second step. However, we are given that the first ionization is more significant than the second, so we will use the concentration of H3O+ directly to calculate the pH.

Formula: pH = -log[H3O+]

Calculation: pH = -log(1.2 × 10—4) = 3.92 (rounded to two decimal places)

Note that for this calculation we considered only the dominant ionization process due to the large difference in pKa values. In real situations, both ionizations might need to be considered to calculate the pH accurately, and the assumption of ignoring the second ionization may not always hold.