Question

which of the equalities is true when the ph is 4.22?
h₃a = h₂a⁻
h₃a = ha²⁻
h₂a⁻ = ha²⁻
h₂a⁻ = a³⁻
ha²⁻ = a³⁻

Explanation:

Step1: Recall the Henderson - Hasselbalch equation

For a diprotic acid $\mathrm{H_2A}$, the relevant dissociation steps and Henderson - Hasselbalch equations are considered. When $\mathrm{pH = pK_{a2}}$ for a diprotic acid, the concentrations of $\mathrm{H_2A^ -}$ and $\mathrm{HA^{2 -}}$ are equal. In general, for an acid - base equilibrium, when $\mathrm{pH}$ is equal to the $\mathrm{pK_a}$ value of a particular dissociation step, the concentrations of the acid - form and conjugate - base form involved in that step are equal.

Answer:

$\mathrm{[H_2A^ -]=[HA^{2 -}]}$