Question

Predict whether a solution containing the salt \(\mathrm{K}_{2} \mathrm{HPO}_{4}\) will be acidic, neutral, or basic.

Short answer

The solution will be basic.

Step-by-step solution

Identify the Components

The salt \(\mathrm{K}_{2}\mathrm{HPO}_{4}\) consists of potassium ions (\(\mathrm{K}^{+}\)) and hydrogen phosphate ions (\(\mathrm{HPO}_{4}^{2-}\)). We need to consider the acid-base properties of these ions.

Analyze Cation's Acid-Base Property

Potassium ions (\(\mathrm{K}^{+}\)) are the cations involved. They are derived from a strong base, potassium hydroxide (\(\mathrm{KOH}\)), which completely dissociates in water. Thus, \(\mathrm{K}^{+}\) does not affect the acidity or basicity of the solution as it is neutral.

Analyze Anion's Acid-Base Property

The anion \(\mathrm{HPO}_{4}^{2-}\) can react with water. It is the conjugate base of \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) and can further dissociate to form \(\mathrm{PO}_{4}^{3-}\), generating \(\mathrm{OH}^-\). Thus, it acts as a weak base.

Predict the Solution's Nature

Since \(\mathrm{K}^{+}\) is neutral and \(\mathrm{HPO}_{4}^{2-}\) can act as a base by generating \(\mathrm{OH}^-\) ions, the solution will be basic overall.

Key concepts

Potassium Ion

Potassium ions, represented as \( \mathrm{K}^+ \), play a key role in determining the nature of a solution. These ions originate from potassium hydroxide (\( \mathrm{KOH} \)), a strong base that dissociates completely in water. This complete dissociation means that every \( \mathrm{KOH} \) molecule breaks apart into \( \mathrm{K}^+ \) ions and \( \mathrm{OH}^- \) ions upon dissolving. Because \( \mathrm{K}^+ \) ions are derived from a strong base, they are considered neutral species in solution.This neutrality arises because \( \mathrm{K}^+ \) does not interact with water to form hydronium (\( \mathrm{H}_3\mathrm{O}^+ \)) or hydroxide ions (\( \mathrm{OH}^- \)). Consequently, potassium ions, by themselves, have no impact on the acidity or basicity of the solution. Therefore, when assessing the overall pH of a solution containing \( \mathrm{K}^+ \), one only needs to consider the effect of the other ions present.

Hydrogen Phosphate Ion

Hydrogen phosphate ions, \( \mathrm{HPO}_4^{2-} \), are fascinating entities in the realm of acid-base chemistry. These ions are derived from phosphoric acid and possess unique properties that influence the pH of a solution. As an anion, \( \mathrm{HPO}_4^{2-} \) is capable of participating in proton transfer reactions.When introduced into water, this ion can either gain a proton to form \( \mathrm{H}_2\mathrm{PO}_4^- \) or lose a proton to become \( \mathrm{PO}_4^{3-} \). In the latter scenario, \( \mathrm{HPO}_4^{2-} \) acts as a weak base by accepting a proton and generating hydroxide ions (\( \mathrm{OH}^- \)).This ability to produce \( \mathrm{OH}^- \) ions means that \( \mathrm{HPO}_4^{2-} \) can increase the basicity of a solution. Therefore, in a solution containing these ions, you can expect a tendency towards a higher pH value, indicative of a basic environment.

Conjugate Base

The notion of a conjugate base is central to understanding acid-base reactions. In this context, \( \mathrm{HPO}_4^{2-} \) serves as the conjugate base of \( \mathrm{H}_2\mathrm{PO}_4^- \). To grasp this concept, remember that a conjugate base is what remains of an acid after it donates a proton.In the case of \( \mathrm{H}_2\mathrm{PO}_4^- \), when it loses a proton, it forms \( \mathrm{HPO}_4^{2-} \). Thus, \( \mathrm{HPO}_4^{2-} \) can potentially accept a proton to regenerate \( \mathrm{H}_2\mathrm{PO}_4^- \), illustrating a classic acid-base equilibrium.This notion of equilibrium is important because it clarifies why \( \mathrm{HPO}_4^{2-} \) can act as a weak base. Its ability to revert to \( \mathrm{H}_2\mathrm{PO}_4^- \) by accepting a proton is what imbues it with basic properties. These reactive features make the hydrogen phosphate ion a key player in creating a solution's basic character.

pH Prediction

Predicting pH is a fundamental aspect of chemistry, particularly in understanding how a solution behaves. In the case of \( \mathrm{K}_2\mathrm{HPO}_4 \), predicting whether the solution is acidic, neutral, or basic involves evaluating the contributions of each ion.First, the neutrality of the \( \mathrm{K}^+ \) ion simplifies the process by removing it as a factor in altering the solution's pH. This neutrality means the focus should solely be on the \( \mathrm{HPO}_4^{2-} \) ions.Given that \( \mathrm{HPO}_4^{2-} \) acts as a weak base, it has the ability to create \( \mathrm{OH}^- \) ions in solution, which pushes the pH above 7, making it basic. Understanding the properties of \( \mathrm{HPO}_4^{2-} \) is crucial here. It is the primary influencer that shifts the solution to a basic pH.Therefore, when faced with the task of predicting the pH of such a solution, knowing the behavior of each ion allows you to conclude accurately. This knowledge highlights the fundamental predictive ability in chemistry, where analyzing components leads to understanding overall solution behavior.