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Chapter 16: Problem 75

Predict whether a solution containing the salt \(\mathrm{K}_{2} \mathrm{HPO}_{4}\) will be acidic, neutral, or basic. (Hint: You need to consider both the ionization and hydrolysis of \(\mathrm{HPO}_{4}^{2-} .\)

Short Answer

Expert verified
The solution of \(\mathrm{K}_{2}\mathrm{HPO}_{4}\) in water will be acidic.

Step by step solution

01

Identify the Anion and Cation

The given salt is \(\mathrm{K}_{2}\mathrm{HPO}_{4}\). The cation in the salt is K+ (Potassium) and the anion is \(\mathrm{HPO}_{4}^{2-}\) (Hydrogen phosphate).
02

Ionization of the Salt

When the salt dissolves in water, it ionizes into its cation and anion. \(\mathrm{K}_{2}\mathrm{HPO}_{4}\) \(\rightarrow\) 2K+ + \(\mathrm{HPO}_{4}^{2-}\)
03

Analyzing the Cation K+

Potassium ion (K+) is the cation of a strong base (Potassium hydroxide, KOH). It doesn’t react with water, so it will not affect the pH of the solution.
04

Analyzing the Anion \(\mathrm{HPO}_{4}^{2-}\)

The anion here is HPO4^2-, which can act as both a weak base and a weak acid because it is amphoteric. When it reacts with water, there will be two reactions happening, and these are as follows: \[ \mathrm{HPO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{H}_{2}\mathrm{O}_{3}^{4-} + \mathrm{H}_{3}\mathrm{O}^{+} \] (acting as a weak acid) and \[ \mathrm{HPO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{H}_{2}\mathrm{PO}_{4}^{-} + \mathrm{OH}^{-} \] (acting as a weak base).
05

Determining the Dominant Reaction

The HPO4^2- ion can increase the concentration of either H3O+ or OH- depending on whether it acts more like a base or an acid. Comparing the Ka (acid dissociation constant) and Kb (base dissociation constant) values for this ion, if Ka > Kb, it acts more like an acid and vice versa. For \(\mathrm{HPO}_{4}^{2-} ,\) the Ka value (6.2x10^-8) is less than the Kb value (2.1x10^-13). Therefore, it acts predominantly as an acid.
06

Determining the Solution's Acidity or Basicity

Since the cation in the solution does not affect the pH and the anion acts as an acid, the solution will be acidic.

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Most popular questions from this chapter

List the factors on which the \(K_{\mathrm{a}}\) of a weak acid depends.

Hydrocyanic acid (HCN) is a weak acid and a deadly poisonous compound that, in the gaseous form (hydrogen cyanide), is used in gas chambers. Why is it dangerous to treat sodium cyanide with acids (such as HCl) without proper ventilation?

Which of the following is the stronger base: \(\mathrm{NF}_{3}\) or \(\mathrm{NH}_{3} ?\) (Hint: \(\mathrm{F}\) is more electronegative than \(\left.\mathrm{H} .\right)\)

Which of the following is a stronger base: \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) ? (Hint: The \(\mathrm{N}-\mathrm{H}\) bond is stronger than the \(\mathrm{P}-\mathrm{H}\) bond. \()\)

Define salt hydrolysis. Categorize salts according to how they affect the \(\mathrm{pH}\) of a solution.
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