Below is a representation of an aqueous solution of a weak acid HA at equilibrium.(Each circle represents 1.0 ×10^-3 mol of atoms,and the volume of the box is 1.0 L.Solvent water molecules are not shown for clarity.) What is the pH of the solution? A)2.4 B)3.0 C)2.1 D)11.0 E)11.6

Question

Quizplus · Accepted Answer

The pH of a solution of a weak acid can be calculated using the expression pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. In this case, the weak acid is represented by the circles, and the conjugate base (A-) is represented by the dots. From the diagram, we can see that the concentration of the conjugate base is roughly 1/10th the concentration of the acid, so [A-]/[HA] is approximately equal to 0.1. The pKa of the weak acid can be estimated to be around 4.7 based on the pH scale shown in the diagram. Plugging these values into the pH expression gives pH = 4.7 + log(0.1) = 3.

Below Is a Representation of an Aqueous Solution of a Weak