Chemical Equilibrium Complex

Question

Calculate the K_a1, K_a2, and K_a3 of Ca₃(PO₄)₂ with a pH of 8.00

Answer 1

Good question! To put it simply, Ca₃(PO₄)₂ breaks apart in water to release phosphate ions (PO₄^3-). These phosphate ions can grab hydrogen ions (H⁺) from water, forming other molecules like HPO4^2-, H₂PO₄^-, and eventually H3PO4​ (phosphoric acid). Each of these steps where we add a subsequent H+ has its own Ka value, which tells us how strongly each molecule is willing to give off those hydrogen ion(s). These Ka values are constants, so we usually look them up because they don’t change with pH. For phosphoric acid, the values are about: Ka₁ = 7.1 × 10⁻³, Ka₂ = 6.3 × 10⁻⁸, and Ka₃ = 4.5 × 10⁻¹³. Just as an FYI, if the pH is 8.00, the solution is slightly basic, so most of the phosphate in the water would exist as HPO4^2-. But just knowing the pH isn’t enough to calculate Ka, as we’d need more info (like the actual concentrations of all the different phosphate forms) to do that!