Final answer:
The solution that is 0.50M nitrous acid and 0.50M sodium nitrite would show the least change in pH upon addition of 3.0 mL of 1.0 M KOH.
Step-by-step explanation:
To determine which solution would show the least change in pH upon addition of 3.0 mL of 1.0 M KOH, we need to consider the initial pH of each solution and the reaction that occurs when KOH is added. The equation for the reaction of KOH with nitrous acid (HNO2) is:
HNO2 (aq) + KOH (aq) → H2O (l) + KNO2 (aq)
Nitrous acid (HNO2) is a weak acid, so it does not completely ionize in water. The initial pH of a solution depends on the initial concentration of HNO2. The solution with the least change in pH upon addition of KOH would be the one with the highest initial concentration of HNO2. Among the given options, the solution that is 0.50 M nitrous acid and 0.50 M sodium nitrite would have the highest initial concentration of HNO2 and therefore show the least change in pH.