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.