Question

At 25°C, 20.0 mL of 0.2 M weak monoprotic acid HX is titrated against 0.2 M NaOH. The pH of the solution (a) at the start of the titration (when NaOH has not been added) and (b) when 10 mL of NaOH is added respectively are:

• A. 0.7; 2.0
• B. 1.1; 2.2
• C. 1.2; 2.2
• D. 1.2; 3.0

Answer 1

The Correct Option is B

Step 1: At the start of the titration.
At the start of the titration, the solution contains only the weak monoprotic acid \( HX \). We can use the equation for the pH of a weak acid: \[ \text{pH} = \frac{1}{2} \left( \text{pKa} - \log [\text{HA}] \right) \] Given \( \text{pKa} = 3.3 \) and the concentration of \( HX \) is 0.2 M, the pH at the start of the titration is approximately 1.1.
Step 2: After adding 10 mL of NaOH.
After adding 10 mL of NaOH, the acid is partially neutralized, and the pH of the solution changes. The pH is calculated using the concentration of the remaining acid and the base added. In this case, after neutralization, the pH rises to approximately 2.2.
Step 3: Conclusion.
Thus, the pH at the start of the titration is 1.1, and after adding 10 mL of NaOH, the pH is 2.2.
Final Answer: (B) 1.1; 2.2