What is the pH of a 0.85 M HClO4 (perchlorate acid, a strong acid) solution?
The correct answer and explanation is:
The pH of a 0.85 M HClO₄ (perchloric acid) solution is 0.070.
Explanation:
Perchloric acid (HClO₄) is a strong acid, meaning it dissociates completely in water. When a strong acid like HClO₄ dissolves in water, it releases hydrogen ions (H⁺) or protons, which are responsible for the acidity of the solution. The dissociation equation for HClO₄ is:
HClO₄(aq)→H⁺(aq)+ClO₄−(aq)\text{HClO₄} (aq) \rightarrow \text{H⁺} (aq) + \text{ClO₄}⁻ (aq)
For a strong acid, the concentration of hydrogen ions ([H⁺]) is equal to the concentration of the acid. In this case, the concentration of HClO₄ is 0.85 M, so the concentration of hydrogen ions is also 0.85 M.
The pH is a measure of the concentration of hydrogen ions in a solution and is defined as: pH=−log[H⁺]\text{pH} = -\log[\text{H⁺}]
Substitute the concentration of hydrogen ions into the equation: pH=−log(0.85)\text{pH} = -\log(0.85)
Using a calculator: pH=−(−0.070)=0.070\text{pH} = -(-0.070) = 0.070
So, the pH of the solution is 0.070.
Additional Notes:
- Strong acids like HClO₄ dissociate completely, meaning there is no need to consider partial dissociation as is the case with weak acids.
- The pH scale ranges from 0 to 14, with 7 being neutral. A pH below 7 indicates an acidic solution, and a pH below 1 is strongly acidic.
- A pH of 0.070 indicates a very acidic solution, which is typical for strong acids at concentrations above 0.1 M.