12.65 Determine whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7. If pH 7 or pH 7, write a chemical equation to justify your answer. (a) NH4Br; (b) Na2CO3; (c) KF; (d) KBr; (e) AlCl3; (f) Cu (NO3) 2.

a. pH > 7; b. pH > 7; c. pH = 7; d. pH = 7; e. pH < 7; f. pH < 7

Explanation:

a. NH₄Br → NH₄⁺ + Br⁻

The ammonium is the conjugate strong acid from a weak base, so it will react in water. Bromine, is the conjugate weak base from a strong acid and it does not react in water. As the NH₄⁺ react as this:

NH₄⁺ + H₂O ⇄ NH₃ + OH⁻ (pH > 7; brings hydroxyls to the medium)

b. Na₂CO₃ → 2Na⁺ + CO₃⁻²

The sodium cation is the conjugate weak acid from a strong base, in water in does not react. The carbonate is the conjugate strong base from a weak acid, so it can react in water as this:

CO₃⁻² + H₂O ⇄ HCO₃⁻ + OH⁻

HCO₃⁻ + H₂O ⇄ H₂CO₃ + OH⁻ (pH > 7; brings hydroxyls to the medium)

c. KF → K⁺ + F⁻

Both ions come from a base and a strong acid, so they will be the weak conjugate base and acid. They will not have a reaction in the water, so the solution will be neutral

d. KBr → K⁺ + Br⁻ (same as before)

e. AlCl₃ → Al³⁺ + 3Cl⁻

Cl⁻ is a conjugate weak base, from a strong acid and it doesn't react in water. But the Al³⁺ which comes from the Al (OH) ₃ is amphoteric, it can act as an acid or a base. In this case, you have a complex and the solution will be acid.

Al³⁺ + H₂O ⇄ Al (OH) ²⁺ + H⁺ (it brings protons to the medium, pH < 7)

f. Cu (NO₃) ₂ → Cu²⁺ + 2NO₃⁻

Ion Cu²⁺ comes from the Cu (OH) ₂, a weak base so it is a conjugate strong acid. The nitrate, comes from a strong acid (nitric), so it is the conjugate weak base of it, it won't react in water.

Cu²⁻ + H₂O ⇄ Cu (OH) ⁺ + H⁺

You will have a complex, and you are bringing protons to the medium, so pH < 7