Final answer:
To find the concentration of H3O+ in a 0.3 M solution of oxalic acid, we use the first pKa to calculate the Ka and then apply the ICE table approach or a quadratic equation to solve for the equilibrium concentration of hydronium ions.
Step-by-step explanation:
To calculate the concentration of H3O+ ions in a 0.3 M solution of oxalic acid, which is a diprotic acid with the given pKa values (pKa1 = 1.25; pKa2 = 3.81), we would typically use the Henderson-Hasselbalch equation. However, because the question pertains to the concentration of hydronium ions directly, and assuming that the first dissociation is the main contributor to the H3O+ concentration due to the significantly lower pKa1 compared to pKa2, we can simplify the approach.
First, we compute the acid dissociation constant (Ka) for the first dissociation from the provided pKa1, using the relation Ka = 10^(-pKa). After that, using the Ka value and the initial concentration of oxalic acid, we can set up an ice table (Initial, Change, Equilibrium) or a quadratic equation to solve for the hydronium ion concentration at equilibrium.
Given the question does not require the details of these computations but only definitions and basic understanding of the process, we do not compute the actual concentration here. Nevertheless, these steps would guide one through the calculation if needed.