15.10
Determining the pH of Salt Solutions
The pH of a salt solution is determined by its component anions and cations. Salts that contain pH-neutral anions and the hydronium ion-producing cati…
Salt solutions can be acidic, basic, or neutral. The individual ions can be examined to determine if the pH of its solution will be below, above, or equal to seven.
Salts that contain pH-neutral cations and anions form neutral solutions. These salts are composed of the counterions of a strong acid and a strong base. A pure aqueous solution of potassium nitrate will have a pH of 7, as neither potassium nor nitrate has acidic or basic properties.
Salts that contain a cation that is the counterion of a weak base and an anion that is the counterion of a strong acid will have a pH less than 7. In a solution of ammonium bromide, the bromide ions are pH neutral, whereas the ammonium ions are acidic and donate protons to the water molecules.
Salts that contain a cation that is the counterion of a strong base and an anion that is the counterion of a weak acid will have a pH greater than 7. In a sodium acetate solution, the sodium ions are pH neutral, whereas the acetate ions are basic and accept protons from the water molecules.
Some salts contain a cation and anion that are counterions of a weak acid and weak base, respectively. The solution will be acidic if the Ka of the cation is larger than the Kb of the anion; basic if the Kb of the anion is larger than the Ka of the cation; or neutral if the Ka and the Kb are equal.
In a solution of ammonium nitrite, the Ka for ammonium is higher than the Kb for nitrite ions; therefore, the solution will be acidic.
The exact pH of a salt solution containing an acidic or basic ion can be determined using the dissociation constant for the conjugate acid or base of that ion.
The pH of 0.35 M sodium cyanide can be determined by calculating the Kb of cyanide from the Ka of its conjugate acid - hydrocyanic acid - and setting up an ICE table to determine the equilibrium concentration.
As sodium cations do not react with water, they do not affect the pH and can be omitted. In contrast, cyanide ions accept protons from water molecules and generate hydroxide ions.
The Kb for cyanide ions can be determined using the equation: Ka × Kb = KW. As the Ka for hydrocyanic acid is 4.90 × 10−10, the Kb for cyanide ions is 2.04 × 10−5.
The Kb is equal to the concentration of hydrocyanic acid times the concentration of hydroxide ions divided by the concentration of cyanide ions.
An ICE table can be constructed to express the initial and equilibrium concentrations. Substituting the equilibrium concentrations into the expression, Kb equals x times x divided by 0.35, which can be verified later by the 5% rule.
Upon solving the equation, x equals 2.7 × 10−3 molar. As x is only 0.77% of 0.35 M, the approximation 0.35 minus x is equal to 0.35 is valid.
The pOH of the solution can be calculated by taking the negative log of 2.7 × 10−3 M, which equals 2.57.
The pH of the solution is determined using the equation pH plus pOH is equal to 14; therefore, the pH of this sodium cyanide solution is 11.43.
View the full transcript and gain access to JoVE Core videos
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.