Problem 190 - Introductory Chemistry Atoms First

Question

Without calculating \(\mathrm{pH}\) values, list these solutions in order of acidity, from lowest \(\mathrm{pH}\) to highest \(\mathrm{pH}\) : (a) \(0.10 \mathrm{M} \mathrm{LiNO}_{3}\) (b) \(0.10 \mathrm{M} \mathrm{NaF}\) (c) \(0.10 \mathrm{M} \mathrm{KOH}\) (d) \(0.10 \mathrm{M} \mathrm{HCN}\) (e) \(0.10 \mathrm{M} \mathrm{HNO}_{3}\) (f) \(0.20 \mathrm{M} \mathrm{HCl}\)

Step-by-Step Solution

Answer

The order of acidity from lowest to highest pH for the given solutions is: (f) \(0.20 \mathrm{M} \mathrm{HCl}\) > (e) \(0.10 \mathrm{M} \mathrm{HNO}_{3}\) > (d) \(0.10 \mathrm{M} \mathrm{HCN}\) > (a) \(0.10 \mathrm{M} \mathrm{LiNO}_{3}\) > (b) \(0.10 \mathrm{M} \mathrm{NaF}\) > (c) \(0.10 \mathrm{M} \mathrm{KOH}\).

Step 1: Identify the Acid-Base Properties

First, let's analyze each solution and classify them as containing strong acids, weak acids, strong bases, or weak bases: (a) \(0.10 \mathrm{M} \mathrm{LiNO}_{3}\) - LiNO3 is a soluble salt, and neither of its ions has acidic or basic properties, so it does not affect the acidity. (b) \(0.10 \mathrm{M} \mathrm{NaF}\) - NaF is the salt of a weak acid (HF) and a strong base (NaOH). Therefore, the solution tends to be basic (due to the F- ion). (c) \(0.10 \mathrm{M} \mathrm{KOH}\) - KOH is a strong base. (d) \(0.10 \mathrm{M} \mathrm{HCN}\) - HCN is a weak acid. (e) \(0.10 \mathrm{M} \mathrm{HNO}_{3}\) - HNO3 is a strong acid. (f) \(0.20 \mathrm{M} \mathrm{HCl}\) - HCl is a strong acid, but with higher concentration than HNO3.

Step 2: Put the Solutions in Order of Increasing Acidity

Now, let's order the solutions based on their acid-base properties: 1. Strong acids: these have the lowest pH values. Here, we have two - (e) and (f). However, (f) has a higher concentration than (e), so it will have a lower pH value. 2. Weak acid: these have higher pH values than strong acids, but lower than neutral or basic solutions. The only weak acid we have is (d). 3. Neutral solution: These have a pH value around 7. We have only (a). 4. Basic solution: these have higher pH values than neutral solutions. Here, we have one basic solution - (b). 5. Strong base: these have the highest pH values. The only strong base we have is (c). Based on this classification, the order of acidity (from lowest to highest pH) will be: (f) > (e) > (d) > (a) > (b) > (c).

Key Concepts

Strong Acids

Strong acids are substances that completely dissociate in water, releasing all of their hydrogen ions (H+) into the solution. This makes their solutions very acidic, having low pH values. For example, in the original exercise, we mentioned HNO3 and HCl as strong acids. When either is dissolved in water, they fully split into their ions:

• HNO3 breaks into H+ and NO3-
• HCl splits into H+ and Cl-

Because they release more H+ ions, the resulting solutions are very low in pH, signifying high acidity. It is crucial to identify strong acids in any acidity-related exercise since they will be at the lower end of the pH scale compared to weak acids, neutral, or basic substances.

Weak Acids

Unlike strong acids, weak acids do not fully dissociate in water. Instead, only some of their molecules release hydrogen ions, which makes them only partially ionized. An example of a weak acid from the exercise is HCN. When dissolved, it remains mostly as HCN in the solution, with relatively few HCN molecules turning into H+ and CN- ions:

• HCN ⇌ H+ + CN-

Since weak acids produce fewer H+ ions than strong acids, their solutions are not as acidic, possessing higher pH values relative to strong acids. Understanding the difference in ionization between strong and weak acids helps in predicting their relative acidity in solution.

Strong Bases

Strong bases are the opposite of strong acids; they completely dissociate in water to release hydroxide ions (OH-), making the solution highly basic. A prominent example featured in the exercise is KOH. Upon dissolving, KOH separates entirely into K+ and OH- ions:

• KOH → K+ + OH-

This complete dissociation allows the solution to have a very high pH, indicating strong basicity. Recognizing strong bases is essential in ordering substances by acidity because they will always have higher pH values compared to acids and even most weak bases.

Weak Bases

Weak bases only partially dissociate in water, releasing fewer hydroxide ions compared to strong bases. This leads to less basic solutions with relatively lower pH levels than strong bases. In the exercise, NaF is an example of a compound that contains a weak base. In water, the fluoride ion (F-) reacts slightly with water to produce a small amount of OH-:

• F- + H2O ⇌ HF + OH-

As fewer OH- ions are released, the resulting solution is weakly basic. In any given list of substances, weak bases will generally be found with pH values closer to neutral than strong bases, further clarifying their relative position when categorizing acids and bases.