What is K_a for a weak monoprotic acid if a 0.020 M solution of the acid has a pH of 3.28 at 25°C?

Question

Quizplus · Accepted Answer

The pH of a weak monoprotic acid can be calculated using the following equation: pH = 0.5(pKa - log[H+]) where pKa is the acid dissociation constant and [H+] is the hydrogen ion concentration. To find pKa, we need to rearrange the equation to: pKa = pH + log[H+]/0.5 We know that the pH of the solution is 3.28, so we can substitute that in: pKa = 3.28 + log[H+]/0.5 We also know that the concentration of the acid is 0.020 M, so we can use the dissociation equation to calculate [H+]: Ka = [H+][A-]/[HA] We can assume that since the acid is weak, most of the acid remains undissociated, so we can say that [HA] ≈ [acid] = 0.020 M. We also know that [A-] ≈ [H+], since the acid dissociates to form one H+ ion and one A- ion. So we can say that [H+] ≈ 0.020 M. Now we can substitute [H+] into the equation for pKa: pKa = 3.28 + log(0.020)/0.5 pKa = 3.28 + 0.70 pKa = 3.98 Finally, we can use the equation for Ka to find K_a: Ka = 10-_a 10-_a = 10-^-4 - this is because the question asks for the answer in scientific notation (i.e. with 1 significant figure). Solving for _a: _a = ^-4 _a = 1.4 × 10^-5 Therefore, the answer is B.

What Is Ka for a Weak Monoprotic Acid If a 0.020

What Is K_a for a Weak Monoprotic Acid If a 0.020