15.10
Determining the pH of Salt Solutions
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Q1: Why is a potassium nitrate solution neutral?
Potassium nitrate forms a neutral solution because both potassium and nitrate ions are pH-neutral. Potassium is the counterion of a strong base, and nitrate is the counterion of a strong acid. Neither ion reacts with water to produce hydronium or hydroxide ions, resulting in a pH of 7.
Q2: How do you determine if a salt solution is acidic or basic?
Examine the component ions to identify their origins. If the cation comes from a weak base and the anion from a strong acid, the solution is acidic. If the cation comes from a strong base and the anion from a weak acid, the solution is basic. When both ions are weak, compare their Ka and Kb values using relative strengths of conjugate acid base pairs to determine overall pH.
Q3: Why is an ammonium bromide solution acidic?
Ammonium bromide is acidic because bromide ions are pH-neutral, but ammonium ions are acidic. The ammonium cation donates protons to water molecules, producing hydronium ions and lowering the pH below 7. Bromide, as the counterion of a strong acid, does not affect pH.
Q4: What makes a sodium acetate solution basic?
Sodium acetate forms a basic solution because sodium ions are pH-neutral while acetate ions are basic. Acetate ions, the counterion of a weak acid, accept protons from water molecules and generate hydroxide ions, raising the pH above 7. This hydrolysis reaction increases solution basicity.
Q5: How do you calculate the pH of a salt solution containing a basic anion?
First, derive the Kb of the anion from the Ka of its conjugate acid using Ka × Kb = Kw. Then construct an ICE table to find equilibrium concentrations. Substitute values into the Kb expression and solve for the hydroxide ion concentration. Calculate pOH from this concentration, then use pH + pOH = 14 to find pH.
Q6: Why can spectator ions be omitted when calculating salt solution pH?
Spectator ions like sodium or potassium do not react with water and do not affect pH. Only ions that undergo hydrolysis—accepting or donating protons—influence the solution's acidity or basicity. Omitting unreactive ions simplifies calculations without affecting the final pH result.
Q7: What does the 5% rule verify when solving salt solution pH problems?
The 5% rule checks whether the approximation that x is negligible compared to the initial concentration is valid. If x is less than 5% of the initial concentration, the simplified equation is accurate. For example, if x equals 2.7 × 10−3 M and the initial concentration is 0.35 M, then x is 0.77%, confirming the approximation's validity.