Question

Which of the following mixtures has the lower \(\mathrm{pH}\) ? (a) Equal volumes of \(0.10 \mathrm{M} \mathrm{HI}\) and \(0.10 \mathrm{M} \mathrm{KOH}\) (b) Equal volumes of \(0.10 \mathrm{M} \mathrm{HI}\) and \(0.10 \mathrm{M} \mathrm{NH}_{3}\)

Short answer

Mixture (b) has the lower pH.

Step-by-step solution

Understanding the Problem

We are given two mixtures, each created using equal volumes of different substances, and we need to determine which mixture has a lower pH.

Analyze Mixture (a)

Mixture (a) combines equal volumes of HI, a strong acid, and KOH, a strong base. Since both are strong and equal in concentration and volume, they neutralize each other completely, forming water and potassium iodide (KI). The pH of the resulting solution is neutral, which is around 7.

Analyze Mixture (b)

Mixture (b) combines equal volumes of HI, a strong acid, and NH3, a weak base. The strong acid will not be completely neutralized as NH3 cannot react with all H+ ions. The resulting solution will still be acidic, and thus have a lower pH than mixture (a).

Conclusion

Since the pH for mixture (a) is neutral at 7 and mixture (b) remains acidic due to incomplete neutralization, mixture (b) has the lower pH.

Key concepts

Acid-Base Reactions

Acid-base reactions involve the interaction of acids and bases which results in the formation of water and a salt. These reactions are fundamental in chemistry and play a pivotal role in the dynamics of many natural and industrial systems. In an acid-base reaction, an acid donates a proton (\( ext{H}^+ \)) to a base. The base, in turn, accepts this proton. This transfer of protons is a characteristic feature of these reactions.
Acids and bases can be identified by their properties:

• Acids typically taste sour and turn blue litmus paper red.
• Bases usually taste bitter and turn red litmus paper blue.

The reaction between hydroiodic acid (HI) and potassium hydroxide (KOH) in mixture (a) is an example of an acid-base reaction. Likewise, the reaction between HI and ammonia (\( ext{NH}_3 \)) in mixture (b) is another instance, though with different characteristics and outcomes. Understanding the behavior of acids and bases in these reactions is essential for predicting the pH of the resulting solutions.

Neutralization

Neutralization is a specific type of acid-base reaction. It occurs when an acid and a base react to form water and a salt, effectively canceling out the pH of each other. During neutralization, the \( ext{H}^+ \) ion from the acid combines with the \( ext{OH}^- \) ion from the base to produce water (H₂O), while the remaining ions form a salt.
Key points about neutralization:

• Neutralization results in a solution that is not acidic nor basic if both acid and base are equimolar and strong.
• It is the principle used to determine the endpoint in titrations.
• The pH of a fully neutralized solution is generally around 7, which is neutral on the pH scale.

In the example of mixture (a), HI and KOH, being strong and equimolar, undergo complete neutralization producing water and the salt potassium iodide (\( ext{KI} \)). This leads to a neutral pH of about 7.

Strong Acids and Bases

Strong acids and bases are substances that completely dissociate into ions in an aqueous solution. This complete dissociation is why they are considered strong, as they consistently release more \( ext{H}^+ \) or \( ext{OH}^- \) ions.
Examples of strong acids include:

• Hydroiodic acid (\( ext{HI} \))
• Hydrochloric acid (\( ext{HCl} \))

And examples of strong bases include:

• Sodium hydroxide (\( ext{NaOH} \))
• Potassium hydroxide (\( ext{KOH} \))

For mixture (a), both HI and KOH are strong, leading to complete neutralization. This results in no excess \( ext{H}^+ \) or \( ext{OH}^- \) ions, thus a neutral pH. Understanding the strong nature of these substances helps in predicting the outcomes of reactions and their effects on pH.

Weak Bases

Weak bases do not fully dissociate in solution, contrasting with strong bases. This means only a small fraction of the base molecules will release \( ext{OH}^- \) ions when dissolved in water. Because of this partial dissociation, weak bases are less effective in neutralizing acids, leading to a lower pH in a reaction involving a strong acid.
Ammonia (\( ext{NH}_3 \)) is a classic example of a weak base. In mixture (b), despite being equimolar with HI, the weak nature of \( ext{NH}_3 \) means it cannot fully neutralize the strong acid. An excess of \( ext{H}^+ \) ions remains, making the solution more acidic.Important characteristics of weak bases:

• They do not fully dissociate in water.
• Reactions with strong acids leave the solution with a residual acidity.
• Understanding their weak dissociation is vital for predicting reaction outcomes.

By keeping these points in mind, one can better understand why mixture (b) is more acidic compared to mixture (a).