Question

Arrange the following \(0.10 \mathrm{M}\) solutions in order of most acidic to most basic. \(\mathrm{KOH}, \mathrm{KNO}_{3}, \mathrm{KCN}, \mathrm{NH}_{4} \mathrm{Cl}, \mathrm{HCl}\)

Short answer

The final order of the solutions, from most acidic to most basic, is: \(\mathrm{HCl}\), \(\mathrm{NH}_{4} \mathrm{Cl}\), \(\mathrm{KNO}_{3}\), \(\mathrm{KCN}\), \(\mathrm{KOH}\).

Step-by-step solution

Identify the type of solution for each compound

In order to categorize them as acidic, basic, or neutral, we'll look at the cations and anions formed when these compounds dissociate in water. \(\mathrm{KOH} \rightarrow \mathrm{K^{+} + OH^{-}}\): Here, \(\mathrm{OH^{-}}\) is produced, making the solution basic. \(\mathrm{KNO}_{3} \rightarrow \mathrm{K^{+} + NO_{3}^{-}}\): Neither \(\mathrm{K^{+}}\) nor \(\mathrm{NO_{3}^{-}}\) react with water, making the solution neutral. \(\mathrm{KCN} \rightarrow \mathrm{K^{+} + CN^{-}}\): \(\mathrm{CN^{-}}\) can gain proton to become \(\mathrm{HCN}\), making the solution basic. \(\mathrm{NH}_{4} \mathrm{Cl} \rightarrow \mathrm{NH_{4}^{+} + Cl^{-}}\): \(\mathrm{NH_{4}^{+}}\) can lose proton to become \(\mathrm{NH_{3}}\), making the solution acidic. \(\mathrm{HCl} \rightarrow \mathrm{H^{+} + Cl^{-}}\): Here, \(\mathrm{H^{+}}\) is produced, making the solution acidic.

Arrange the solutions

After identifying the type of solution for each compound, we'll arrange them in order of most acidic to most basic. There are two acidic solutions: \(\mathrm{NH_{4} \mathrm{Cl}}\) and \(\mathrm{HCl}\). There is one neutral solution: \(\mathrm{KNO}_{3}\). There are two basic solutions: \(\mathrm{KOH}\) and \(\mathrm{KCN}\). Since the concentrations of all solutions are the same (0.10 M), the order of acidity or basicity will be determined based on their respective strength (please seek necessary information if needed). Acidic: \(\mathrm{HCl}\) (strong acid) > \(\mathrm{NH}_{4} \mathrm{Cl}\) (weak acid) Basic: \(\mathrm{KOH}\) (strong base) > \(\mathrm{KCN}\) (weak base)

Final answer

Therefore, the final order of the solutions, from most acidic to most basic, is: \(\mathrm{HCl}\), \(\mathrm{NH}_{4} \mathrm{Cl}\), \(\mathrm{KNO}_{3}\), \(\mathrm{KCN}\), \(\mathrm{KOH}\).

Key concepts

pH Scale

The pH scale is a numerical representation of how acidic or basic a solution is. It ranges from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are basic or alkaline. The lower the pH value, the higher the acidity, and conversely, the higher the pH value, the higher the basicity. The pH scale is logarithmic, which means each whole pH value below 7 is ten times more acidic than the next higher value. For example, a solution with a pH of 3 is ten times more acidic than one with a pH of 4.

The scale is based on the concentration of hydrogen ions (\(H^+\)) in the solution. A higher concentration of \(H^+\) ions results in a lower pH, indicating an acidic solution. On the other hand, a higher concentration of hydroxide ions (\(OH^-\)) results in a higher pH, indicating a basic solution. This is important when evaluating the acidity or basicity of chemical solutions, such as in the original exercise where different solutes were dissolved in water to form solutions of equal molarity but varying pH.

Acid-Base Reactions

Acid-base reactions are chemical reactions that involve the transfer of one or more protons (\(H^+\)) between reactants. In an acid-base reaction, an acid donates a proton and becomes a conjugate base, whereas a base accepts a proton and becomes a conjugate acid. Understanding these reactions is crucial when trying to arrange solutions by acidity or basicity, as shown in the exercise.

For instance, \(NH_{4}Cl\) in water dissociates to form \(NH_{4}^+\) and \(Cl^-\). The \(NH_{4}^+\) ion is an acid because it can donate a proton to water to form \(NH_3\) and \(H_3O^+\), thereby increasing the acidity of the solution. In contrast, \(KCN\) dissociates to form \(K^+\) and \(CN^-\), and the \(CN^-\) ion is a base because it can accept a proton from water. These acid-base properties determine where a solution falls on the pH scale.

Chemical Solution Concentrations

The concentration of a chemical solution refers to the amount of solute that is dissolved in a given quantity of solvent. It is typically expressed in molarity (M), which is moles of solute per liter of solution. In the exercise provided, all solutions have the same concentration of 0.10 M, which simplifies the comparison process because it eliminates concentration as a variable affecting acidity or basicity.

Despite identical concentrations, not all solutions have the same pH. This is because different solutes dissociate to produce different amounts of \(H^+\) or \(OH^-\) ions, which then influence the pH. Strong acids and bases dissociate completely in water, contributing more \(H^+\) or \(OH^-\) ions, respectively, making their solutions more acidic or basic. In contrast, weak acids and bases only partially dissociate, resulting in less extreme pH values. Recognizing the nature of the solute—whether it is a strong or weak acid or base—is vital when assessing the pH of similar concentrations.