Question

Predict whether aqueous solutions of the following substances are acidic, basic, or neutral: (a) ${\mathrm{CrBr}}_3$, (b) LiI, (c) ${\mathrm{K}}_3{\mathrm{PO}}_4$ (d) $\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]\mathrm{Cl}$, (e) ${\mathrm{KHSO}}_4$

Short answer

The aqueous solutions of the given substances have the following properties: a) ${\mathrm{CrBr}}_3$: Neutral, b) LiI: Slightly basic, c) ${\mathrm{K}}_3{\mathrm{PO}}_4$: Basic, d) $\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]\mathrm{Cl}$: Acidic, e) ${\mathrm{KHSO}}_4$: Acidic but slightly less acidic than a solution containing a strong acid.

Step-by-step solution

a) ${\mathrm{CrBr}}_3$

${\mathrm{CrBr}}_3$ is composed of a chromium(III) ion, ${\mathrm{Cr}}^{3+}$, and three bromide ions, ${\mathrm{Br}}^{-}$. ${\mathrm{Cr}}^{3+}$ is a metal ion and does not form a basic solution, and ${\mathrm{Br}}^{-}$ is the conjugate base of a strong acid, $\mathrm{HBr}$. When in solution, it does not hydrolyze or generate any additional ${\mathrm{H}}^{+}$ ions or ${\mathrm{OH}}^{-}$ ions. Thus, the aqueous solution of ${\mathrm{CrBr}}_3$ is neutral.

b) LiI

LiI is composed of a lithium ion, ${\mathrm{Li}}^{+}$, and an iodide ion, ${\mathrm{I}}^{-}$. ${\mathrm{Li}}^{+}$ is an alkali metal ion and can form a basic solution. ${\mathrm{I}}^{-}$ is the conjugate base of a strong acid, $\mathrm{HI}$. When in solution, it does not hydrolyze or generate any additional ${\mathrm{H}}^{+}$ ions or ${\mathrm{OH}}^{-}$ ions. Thus, the aqueous solution of LiI is slightly basic.

c) ${\mathrm{K}}_3{\mathrm{PO}}_4$

${\mathrm{K}}_3{\mathrm{PO}}_4$ is composed of three potassium ions, ${\mathrm{K}}^{+}$, and one phosphate ion, ${\mathrm{PO}}_4^{3-}$. ${\mathrm{K}}^{+}$ is an alkali metal ion and can form a basic solution. ${\mathrm{PO}}_4^{3-}$ is the conjugate base of a weak acid, ${\mathrm{H}}_3{\mathrm{PO}}_4$, so it will hydrolyze and generate additional ${\mathrm{OH}}^{-}$ ions. Thus, the aqueous solution of ${\mathrm{K}}_3{\mathrm{PO}}_4$ is basic.

d) $\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]\mathrm{Cl}$

$\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]\mathrm{Cl}$ is composed of a methylammonium ion, ${\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]}^{+}$, and a chloride ion, ${\mathrm{Cl}}^{-}$. ${\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]}^{+}$ is the conjugate acid of a weak base, ${\mathrm{CH}}_3{\mathrm{NH}}_2$, so it will hydrolyze and generate additional ${\mathrm{H}}^{+}$ ions. ${\mathrm{Cl}}^{-}$ is the conjugate base of a strong acid, $\mathrm{HCl}$, and does not hydrolyze or generate any additional ${\mathrm{H}}^{+}$ ions or ${\mathrm{OH}}^{-}$ ions. Thus, the aqueous solution of $\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]\mathrm{Cl}$ is acidic.

e) ${\mathrm{KHSO}}_4$

${\mathrm{KHSO}}_4$ is composed of a potassium ion, ${\mathrm{K}}^{+}$, and a hydrogen sulfate ion, ${\mathrm{HSO}}_4^{-}$. ${\mathrm{K}}^{+}$ is an alkali metal ion and can form a basic solution. ${\mathrm{HSO}}_4^{-}$ is the conjugate base of a strong acid, ${\mathrm{H}}_2{\mathrm{SO}}_4$, and the conjugate acid of the weak base, ${\mathrm{SO}}_4^{2-}$. It will hydrolyze and generate both ${\mathrm{H}}^{+}$ ions and ${\mathrm{OH}}^{-}$ ions. Since ${\mathrm{K}}^{+}$ ions can form a basic solution and ${\mathrm{HSO}}_4^{-}$ ions can form both acidic and basic solutions, the overall result is that the aqueous solution of ${\mathrm{KHSO}}_4$ is acidic but slightly less acidic than a solution containing a strong acid.

Key concepts

Acidic and Basic Solutions

Aqueous solutions are typically categorized as acidic, basic, or neutral based on the ionic interactions that occur when a substance is dissolved in water.
If a solution is acidic, it means that the concentration of hydrogen ions ($H^{+}$) is greater than that of hydroxide ions ($O{H}^{-}$).
In a basic solution, the concentration of hydroxide ions outnumbers that of hydrogen ions.
Neutral solutions have equal concentrations of $H^{+}$ and $O{H}^{-}$ ions.

• An example of neutral solution: ${\mathrm{CrBr}}_3$, as its ions do not hydrolyze further.
• Examples of basic solutions include compounds like ${\mathrm{K}}_3{\mathrm{PO}}_4$, which generates $O{H}^{-}$ ions through hydrolysis.
• Conversely, solutions like $[{\mathrm{CH}}_3{\mathrm{NH}}_3]\mathrm{Cl}$ are acidic due to the generation of $H^{+}$ ions.

Hydrolysis

Hydrolysis is a chemical reaction between water and another substance, often leading to the breakdown or alteration of the latter.
In the context of aqueous solutions, hydrolysis can alter the pH of the solution by generating $H^{+}$ or $O{H}^{-}$ ions.

• For example, phosphate ions ($P{O}_4^{3-}$ in ${\mathrm{K}}_3{\mathrm{PO}}_4$) undergo hydrolysis to produce hydroxide ions ($O{H}^{-}$).
• The $[{\mathrm{CH}}_3{\mathrm{NH}}_3{]}^{+}$ ion will release $H^{+}$ ions, making the solution more acidic.

Through hydrolysis, compounds can influence whether a solution is acidic or basic.

Conjugate Acid and Base

When an acid donates a proton ($H^{+}$), it forms a conjugate base.
Similarly, when a base accepts a proton, it forms a conjugate acid.
The concept is crucial in acids and bases because it helps determine how substances behave in solution.

• For instance, ${\mathrm{I}}^{-}$ is the conjugate base of the strong acid $\mathrm{HI}$, which does not affect the $O{H}^{-}$ concentration in its aqueous solution.
• The methylammonium ion $[{\mathrm{CH}}_3{\mathrm{NH}}_3{]}^{+}$ found in $[{\mathrm{CH}}_3{\mathrm{NH}}_3]\mathrm{Cl}$ is the conjugate acid of a weak base. It will increase $H^{+}$ concentration when dissolved in water.

Understanding conjugate acids and bases is essential for predicting the pH of solutions.

Chemical Ions

Chemical ions are atoms or molecules that have gained or lost an electron, resulting in a net charge.
They play a significant role in determining the pH of solutions.

• For example, ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Br}}^{-}$ in ${\mathrm{CrBr}}_3$ do not further react with water, leading to a neutral solution.
• In ${\mathrm{KHSO}}_4$, the ${\mathrm{HSO}}_4^{-}$ ion can undergo reactions that influence the acidity of the solution.

By analyzing the nature of the ions in a solution, one can predict whether a solution will be acidic, basic, or neutral.

pH Determination

Determining the pH of a solution involves measuring the concentration of hydrogen ions ($H^{+}$).
pH values range from 0 to 14, with 7 being neutral, values less than 7 indicating acidity, and values greater than 7 indicating basicity.

• For example, the ${\mathrm{K}}_3{\mathrm{PO}}_4$ solution has a pH greater than 7 due to the formation of $O{H}^{-}$ ions.
• In contrast, $[{\mathrm{CH}}_3{\mathrm{NH}}_3]\mathrm{Cl}$ contributes $H^{+}$ ions, resulting in a pH less than 7.

Understanding pH is crucial for grasping how different substances interact in aqueous solutions.