Question:
Phosphoric acid dissociates upto 80% in its \(0.02\text{ M}\) solution at \(25^{\circ }\)C. Calculate the pH of the solution.
Phosphoric acid dissociates upto 80% in its \(0.02\text{ M}\) solution at \(25^{\circ }\)C. Calculate the pH of the solution.
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Treat the acid as tribasic so each dissociated molecule releases three hydrogen ions. Find the ion concentration and then the pH.
Updated On: Nov 12, 2026
- \(1.319\)
- \(1.651\)
- \(1.101\)
- \(1.921\)
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept
\(\text{H}_3\text{PO}_4\) is tribasic. Each dissociated molecule releases three \(\text{H}^+\) ions.
Step 2: Concentration of Hydrogen Ions
Dissociated acid: \(0.02 \times 0.8 = 0.016\text{ M}\)
\([\text{H}^+] = 3 \times 0.016 = 0.048\text{ M}\)
Step 3: pH
\(pH = -\log(0.048) = 2 - \log 4.8 = 2 - 0.681 = 1.319\)
Final Answer:
The pH of the solution is 1.319. \[ \boxed{\text{(A) }1.319} \]
\(\text{H}_3\text{PO}_4\) is tribasic. Each dissociated molecule releases three \(\text{H}^+\) ions.
Step 2: Concentration of Hydrogen Ions
Dissociated acid: \(0.02 \times 0.8 = 0.016\text{ M}\)
\([\text{H}^+] = 3 \times 0.016 = 0.048\text{ M}\)
Step 3: pH
\(pH = -\log(0.048) = 2 - \log 4.8 = 2 - 0.681 = 1.319\)
Final Answer:
The pH of the solution is 1.319. \[ \boxed{\text{(A) }1.319} \]
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