What is the relationship between pKa and the strength of an acid, and the relationship between the pH and the acidity of a solution? A strong acid has a large pKa value, and an acidic solution has a pH =7 A strong acid has a large pKa value, and an acidic solution has a pH >7 A strong acid has a small pKa value, and an acidic solution has a pH =7 A strong acid has a small pKa value, and an acidic solution has a pH <7
The Correct Answer and Explanation is:
The correct answer is: A strong acid has a small pKa value, and an acidic solution has a pH <7.
Explanation:
pKa and Acid Strength:
The strength of an acid is inversely related to its pKa value. The pKa is the negative logarithm of the acid dissociation constant (Ka) of a given acid. Mathematically, it is expressed as:
pKa=−log(Ka)\text{pKa} = -\log(\text{Ka})pKa=−log(Ka)
- A strong acid dissociates almost completely in water, meaning it releases a large amount of hydrogen ions (H⁺). This means that the Ka value for a strong acid is high, and therefore, its pKa value is low (since pKa is the negative log of Ka, and a high Ka gives a low pKa).
- Conversely, a weak acid dissociates less completely, so its Ka value is lower, resulting in a higher pKa value.
So, when an acid has a small pKa value, it is a strong acid, indicating that it dissociates significantly in solution.
pH and Acidity:
The pH of a solution is a measure of the concentration of hydrogen ions (H⁺) in the solution. It is calculated as:
pH=−log[H+]\text{pH} = -\log[\text{H}^+]pH=−log[H+]
- A solution is considered acidic when its pH is less than 7. This occurs because an acidic solution has a higher concentration of hydrogen ions than a neutral solution (which has a pH of 7).
- The pH of a strong acid solution will be less than 7 because a strong acid dissociates almost completely, releasing a high concentration of H⁺ ions.
- On the other hand, a basic solution has a pH greater than 7, with fewer hydrogen ions and more hydroxide ions (OH⁻).
In summary, a strong acid has a low pKa and will result in an acidic solution with pH <7. A weak acid has a higher pKa, and its solution will also have a pH less than 7, but not as low as that of a strong acid.
