The pH of Phosphoric Acid Solution
From the given data, we know that phosphoric acid has a pKa of 2.1 and we need to find the pH at which 75% of phosphoric acid will be in the conjugate base form.
Explanation:
Phosphoric acid (H₃PO₄) is a polyprotic acid that can donate three protons in three sequential reactions:
H₃PO₄ ↔ H₂PO₄⁻ + H⁺ (Ka1) [pKa1 = 2.14]
H₂PO₄⁻ ↔ HPO₄²⁻ + H⁺ (Ka2) [pKa2 = 7.20]
HPO₄²⁻ ↔ PO₄³⁻ + H⁺ (Ka3) [pKa3 = 12.35]
The pKa values indicate the strength of an acid. Lower pKa values mean stronger acids. The Henderson-Hasselbalch equation is a useful tool to calculate the pH of a buffer solution:
pH = pKa + log [conjugate base] / [acid]
Solution:
Given that 75% of the phosphoric acid is in the conjugate base form and 25% is in the acid form, we can calculate the pH as follows:
pH = 2.1 + log(75% / 25%)
pH = 2.1 + log(3)
pH = 2.1 + 0.4771
pH ≈ 2.58
Therefore, at a pH of approximately 2.58, 75% of the phosphoric acid will be in the conjugate base form.