Question

Prediet the \(\mathrm{pH}(>7,<7,\) or \(\approx 7)\) of the aqueous solutions containing the following salts: (a) \(\mathrm{KBr}\) (b) \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) (c) \(\mathrm{BaCl}_{2}\) (d) \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\).

Short answer

(a) The pH of an \(\mathrm{KBr}\) solution will be approximately 7 (neutral), (b) the pH of a \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) solution will be less than 7 (acidic), (c) the pH of a \(\mathrm{BaCl}_{2}\) solution will be approximately 7 (neutral), and (d) the pH of a \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\) solution will be less than 7 (acidic).

Step-by-step solution

Identify if Cation or Anion is Acidic/Basic

Examine the cation and anion in each of the salts. Salts are composed of a cation (positive ion) and anion (negative ion). Identify if any of these are sources of acidity or alkalinity. If neither the cation nor the anion hydrolyze (react), the salt forms a neutral solution with a pH of 7.

Predict pH for \(\mathrm{KBr}\)

\(\mathrm{KBr}\) is composed of KA+ and Br- ions neither of which undergo hydrolysis as K+ is a cation of a strong base (KOH) and Br- is the anion of a strong acid (HBr). Therefore, \(\mathrm{KBr}\) creates a neutral solution with a pH is approximately 7.

Predict pH for \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\)

\(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) is composed of \(\mathrm{Al}^{3+}\) and \(\left(\mathrm{NO}_{3}\right)^-\) ions. The \(\mathrm{Al}^{3+}\) ion is a small, highly-charged metal ion and is acidic. The \(\left(\mathrm{NO}_{3}\right)^-\) ion is the conjugate base of a strong acid, HNO3, and it does not hydrolyze. Since our salt contains a cation that forms acidic solutions, the solution will be acidic with a pH less than 7.

Predict pH for \(\mathrm{BaCl}_{2}\)

\(\mathrm{BaCl}_{2}\) is composed of Ba2+ and Cl- ions neither of which undergo hydrolysis as Ba2+ is a cation of a strong base (Ba(OH)2) and Cl- is the anion of a strong acid (HCl). Therefore, \(\mathrm{BaCl}_{2}\) creates a neutral solution and the pH is approximately 7.

Predict pH for \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\)

\(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\) is composed of \(\mathrm{Bi}^{3+}\) and \(\left(\mathrm{NO}_{3}\right)^-\) ions. The \(\mathrm{Bi}^{3+}\) ion is a small, highly-charged metal ion and is acidic. The \(\left(\mathrm{NO}_{3}\right)^-\) ion is the conjugate base of a strong acid, HNO3, and it does not hydrolyze. Since our salt contains a cation that forms acidic solutions, the solution will be acidic with a pH less than 7.

Key concepts

Acid-Base Properties of Salts

Understanding the acid-base properties of salts is crucial for predicting the pH of their aqueous solutions. Salts are formed from the neutralization reaction between an acid and a base. They consist of cations (positively charged ions) and anions (negatively charged ions). The pH of a salt solution can be influenced by the nature of these ions.

If a salt has a cation that comes from a strong base (like KOH or Ba(OH)₂) and an anion from a strong acid (such as HCl or HBr), it is likely to form a neutral solution with a pH close to 7. This is because neither ion tends to hydrolyze, or react with water, to form additional H⁺ or OH^- ions. For example, KBr and BaCl₂ in our exercise belong to this category, hence their neutral pH.

However, if the cation is from a weak base (like Bi(OH)₃) or the anion is from a weak acid, the salt can be either acidic or basic due to hydrolysis, where the ions react with water and affect the concentration of H⁺ and OH^- ions in the solution. In our exercise, Al(NO₃)₃ and Bi(NO₃)₃ contain metal ions that are acidic, leading to solutions with pH less than 7.

Ion Hydrolysis

Ion hydrolysis is the chemical process in which ions react with water to form new substances, potentially influencing the pH of the solution. When a salt dissolves in water, cations and anions may remain as spectators, or they may participate actively in hydrolysis.

An anion left over from a strong acid, such as NO₃^-, does not hydrolyze because the parent acid, HNO₃, completely dissociates in water. These anions do not affect the pH, as observed with the nitrate salts in our exercise. Conversely, anions from weak acids may accept H⁺ ions from water to reform the weak acid and generate OH^-, making the solution basic.

Similarly, cations from strong bases, like K⁺ and Ba²⁺, do not hydrolyze and don't disturb the pH. In contrast, other metal cations, especially those that are small and highly charged, such as Al³⁺ and Bi³⁺, can donate H⁺ ions to the solution by reacting with water, creating an acidic environment. This concept explains why Al(NO₃)₃ and Bi(NO₃)₃ form acidic solutions.

Strong Acids and Bases

Strong acids and bases are substances that completely dissociate into their constituent ions in aqueous solutions. This complete dissociation means that every molecule of the acid or base breaks apart to contribute H⁺ or OH^- ions to the solution. For strong acids, such as HCl, HBr, and HNO₃, this results in a high concentration of H⁺ ions, and thus a low pH.

Strong bases, like KOH and Ba(OH)₂, fully dissociate to produce OH^- ions, leading to a high pH. In our exercise, the salts KBr and BaCl₂ result in a neutral pH because they originate from the neutralization of strong acids and strong bases.

Hence, when predicting the pH of salt solutions, it's important to recognize whether the ions derived from strong acids and strong bases, as these will not hydrolyze and the resulting solution will be neutral. In contrast, salts involving weak acids or bases can give acidic or basic solutions due to the incomplete dissociation of the original substances and the subsequent hydrlysis of their ions.