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Multiple Choice
Calculate the pH of a 0.205 M KNO2 solution, given that the Ka of nitrous acid (HNO2) is 4.6 × 10^(-4).
A
7.00
B
5.92
C
8.08
D
9.14
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Verified step by step guidance
1
Identify that KNO2 is a salt derived from a weak acid (HNO2) and a strong base (KOH). Therefore, the solution will be basic due to the hydrolysis of the NO2- ion.
Write the hydrolysis equation for the NO2- ion: NO2^- + H2O ⇌ HNO2 + OH^-. This shows that NO2- reacts with water to form HNO2 and OH^-.
Use the relationship between Ka and Kb to find the Kb for NO2-. The formula is Kb = Kw / Ka, where Kw is the ion-product constant of water (1.0 × 10^(-14) at 25°C).
Set up the expression for Kb: Kb = [HNO2][OH^-] / [NO2^-]. Assume that the change in concentration of NO2- due to hydrolysis is small, so [NO2^-] ≈ 0.205 M.
Solve for [OH^-] using the Kb expression, then calculate the pOH of the solution. Finally, use the relationship pH + pOH = 14 to find the pH of the solution.