Problem 219 - Introductory Chemistry Atoms First

Question

\(\mathrm{HClO}_{4}\) is a strong acid, and \(\mathrm{HClO}_{2}\) is a weak acid. If you had a \(1.0 \mathrm{M}\) solution of \(\mathrm{NaClO}_{4}\) and a \(1.0 \mathrm{M}\) solution of \(\mathrm{NaClO}_{2}\), which would have the higher pH? Explain.

Step-by-Step Solution

Answer

The 1.0 M NaClO2 solution would have a higher pH than the 1.0 M NaClO4 solution because HClO2 is a weak acid, producing fewer H3O+ ions in solution than the strong acid HClO4.

Step 1: NaClO4 is the sodium salt of HClO4, which is a strong acid, whereas NaClO2 is the sodium salt of HClO2, which is a weak acid. #Step 2: Write the dissociation equation for each salt#

When NaClO4 and NaClO2 dissociate in water, they form their corresponding acidic species and Na+ ions. The equation for the dissociation of NaClO4 and NaClO2 can be represented by: \( NaClO_4 \rightarrow ClO_4^- + Na^+ \) \( NaClO_2 \rightarrow ClO_2^- + Na^+ \) #Step 3: Determine the degree of dissociation for each salt#

Step 2: For NaClO4, ClO4- comes from a strong acid HClO4, and it is completely dissociated in water. Therefore, the degree of dissociation is 100%. With 1.0 M concentration of NaClO4, the resulting concentration of ClO4- is also 1.0 M. For NaClO2, ClO2- comes from a weak acid HClO2; it does not dissociate completely in water. However, we can assume a significant dissociation, leading to a certain concentration of ClO2- based on the Ka of HClO2. #Step 4: Assess the pH of the solutions and compare#

Since ClO4- has a higher concentration and comes from a strong acid, it contributes more H3O+ ions to the water than ClO2- does, resulting in a lower pH for the NaClO4 solution. Inversely, as NaClO2 comes from a weak acid, there will be fewer H3O+ ions produced by the ClO2- ions; this will result in a higher pH for the NaClO2 solution compared to the NaClO4 solution. Conclusion:

Step 3: Conclusion

The 1.0 M NaClO2 solution would have a higher pH than the 1.0 M NaClO4 solution because HClO2 is a weak acid, producing fewer H3O+ ions in solution than the strong acid HClO4.

Key Concepts

Strong Acids

Strong acids are powerful at releasing hydrogen ions (H\(^+\)) into a solution. They completely dissociate when they dissolve in water. For example,

• Perchloric acid ( \( \mathrm{HClO}_4 \) ) is a strong acid.

This means when \( \mathrm{NaClO}_4 \) dissolves, it releases \( \mathrm{ClO}_4^- \) ions completely, supplying more hydrogen ions and resulting in a low pH solution.
In essence, a strong acid makes the environment more acidic because it contributes more \( \mathrm{H_3O^+} \) ions.

Weak Acids

Weak acids dissociate in water, but not completely. They release fewer hydrogen ions compared to strong acids. An example of a weak acid is chlorous acid (\( \mathrm{HClO}_2 \)).

• Chlorous acid only partially separates into \( \mathrm{ClO}_2^- \) and \( \mathrm{H^+} \) ions.
• This partial dissociation leads to a much lower concentration of \( \mathrm{H^+} \) ions in comparison to strong acids.

Because of this partial dissociation, the solution remains less acidic, creating a higher pH environment than a strong acid solution.

Dissociation

Dissociation refers to the process where compounds break apart into ions when dissolved in water. This concept is key to understanding acids' strength.

• Strong acids like \( \mathrm{HClO}_4 \) dissociate completely, meaning every molecule separates into ions.
• Weak acids, on the other hand, only partially dissociate, leaving many molecules intact.

Complete dissociation results in more hydrogen ions, thus a more acidic solution. The degree of dissociation directly impacts the acidity and pH level of the solution.

Acidic Solutions

The term "acidic solution" refers to a mixture where there is a high concentration of hydrogen ions.

• When a strong acid dissociates, it produces more \( \mathrm{H_3O^+} \) ions, making the solution more acidic.
• Conversely, a weak acid generates fewer \( \mathrm{H_3O^+} \) ions, making it less acidic and increasing the pH.

The balance of these ions determines the acidity level.
For instance, in the given scenario, solutions from salts of strong acids ( \( \mathrm{NaClO}_4 \) ) will always be more acidic compared to those from salts of weak acids ( \( \mathrm{NaClO}_2 \) ).