Question

Classify these aqueous solutions as acidic, neutral, or basic. a. \(\mathrm{HI}(a q)\) b. \(\mathrm{NaCl}(a q)\) c. \(\mathrm{NH}_{4} \mathrm{OH}(a q)\) d. \(\left[\mathrm{H}^{+}\right]=1 \times 10^{-8} \mathrm{M}\) e. \(\left[\mathrm{OH}^{-}\right]=1 \times 10^{-2} \mathrm{M}\) f. \(\left[\mathrm{H}^{+}\right]=5 \times 10^{-7} \mathrm{M}\) g. \(\left[\mathrm{OH}^{-}\right]=1 \times 10^{-12} \mathrm{M}\)

Short answer

a. Acidic; b. Neutral; c. Basic; d. Basic; e. Basic; f. Acidic; g. Acidic.

Step-by-step solution

Classify HI(aq)

Hydroiodic acid, \( \mathrm{HI} \), is a strong acid because it completely dissociates in water to form \( \mathrm{H}^+ \) ions. Therefore, \( \mathrm{HI}(a q) \) is classified as an acidic solution.

Classify NaCl(aq)

Sodium chloride, \( \mathrm{NaCl} \), is a neutral salt. In an aqueous solution, \( \mathrm{NaCl} \) dissociates into \( \mathrm{Na}^+ \) and \( \mathrm{Cl}^- \) ions, neither of which affect the pH of the solution. Hence, \( \mathrm{NaCl}(a q) \) is neutral.

Classify NH4OH(aq)

Ammonium hydroxide, \( \mathrm{NH}_4\mathrm{OH} \), is a weak base because it partially dissociates in water to form \( \mathrm{OH}^- \) ions. Therefore, \( \mathrm{NH}_4\mathrm{OH}(a q) \) is classified as basic.

Classify solution with [H+] = 1 x 10^-8 M

The concentration \( [\mathrm{H}^+] = 1 \times 10^{-8} \mathrm{M} \) suggests a pH greater than 7. Therefore, the solution is slightly basic.

Classify solution with [OH-] = 1 x 10^-2 M

The concentration \( [\mathrm{OH}^-] = 1 \times 10^{-2} \mathrm{M} \) indicates a high concentration of hydroxide ions, suggesting a low pH value. Thus, the solution is basic.

Classify solution with [H+] = 5 x 10^-7 M

The concentration \( [\mathrm{H}^+] = 5 \times 10^{-7} \mathrm{M} \) suggests a pH slightly less than 7, indicating an acidic solution.

Classify solution with [OH-] = 1 x 10^-12 M

The concentration \( [\mathrm{OH}^-] = 1 \times 10^{-12} \mathrm{M} \) is very low, suggesting a low pH value, thus the solution is acidic.

Key concepts

Acidic Solutions

Acidic solutions are characterized by having a higher concentration of hydrogen ions (\(\mathrm{H}^+\)) compared to hydroxide ions (\(\mathrm{OH}^-\)). This results in a pH less than 7. In acidic solutions, substances can dissociate in water to release free hydrogen ions.
For example:

• Hydroiodic acid (\(\mathrm{HI}\)) completely dissociates in water, producing more hydrogen ions, therefore, it forms an acidic solution.
• A solution with \([\mathrm{H}^+] = 5 \times 10^{-7} \mathrm{M}\) completes the criteria of being slightly acidic, as its pH is just under 7.
• A concentration of hydroxide ions lower than hydrogen ions, as with \([\mathrm{OH}^-] = 1 \times 10^{-12} \mathrm{M}\), indicates an acidic solution, providing a low pH value.

Basic Solutions

Basic solutions, or alkaline solutions, have a lower concentration of hydrogen ions compared to hydroxide ions, leading to a pH greater than 7. A classic example of a basic solution is when a substance, such as ammonium hydroxide \(\mathrm{NH}_4\mathrm{OH}\), partially dissociates in water, producing hydroxide ions.
More examples include:

• A solution with a high \([\mathrm{OH}^-] = 1 \times 10^{-2} \mathrm{M}\) concentration is strongly basic due to the surplus of hydroxide ions awaiting to balance the pH.
• A solution where \([\mathrm{H}^+] = 1 \times 10^{-8} \mathrm{M}\) signifies a slightly basic environment, as it hints at a pH greater than 7.

Neutral Solutions

A neutral solution occurs when the concentrations of \(\mathrm{H}^+\) and \(\mathrm{OH}^-\) are equal, resulting in a pH of exactly 7. Water (\(\mathrm{H}_2\mathrm{O}\)) is the perfect example of a neutral solution. A common neutral solution is one that contains sodium chloride (\(\mathrm{NaCl}\)). When dissolved in water, \(\mathrm{NaCl}\) dissociates into \(\mathrm{Na}^+\) and \(\mathrm{Cl}^-\) ions. However, these ions do not affect the concentrations of hydrogen or hydroxide ions, keeping the solution neutral. By not altering the \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) levels, \(\mathrm{NaCl}(aq)\) remains with a pH of 7.

Dissociation in Water

Dissociation is a process by which molecules such as acids, bases, and salts separate into ions in water. This process is essential in determining the pH of a solution.
A few key things about dissociation include:

• Strong acids and bases like hydroiodic acid (\(\mathrm{HI}\)) completely dissociate, producing a high concentration of \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) respectively.
• Weak acids and bases such as ammonium hydroxide (\(\mathrm{NH}_4\mathrm{OH}\)) partially dissociate, which explains the moderate rise in ion concentration.
• Salts like sodium chloride (\(\mathrm{NaCl}\)) dissolve and separate into ions, \(\mathrm{Na}^+\) and \(\mathrm{Cl}^-\), but do not impact solution pH due to the absence of \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) influence.

Understanding how substances dissociate helps in predicting the acidity or basicity of an aqueous solution.