Question

Which of the following when added to 20 mL of a 0.01 M solution of HCl would decrease its pH?

• A. \( 20 \text{ mL of } 0.02 \text{ M HCl} \)
• B. \( 20 \text{ mL of } 0.005 \text{ M HCl} \)
• C. \( 20 \text{ mL of } 0.01 \text{ M HCl} \)
• D. \( 40 \text{ mL of } 0.005 \text{ M HCl} \)

Hint:

To decrease pH: - Increase acid concentration. - Choose stronger acids or higher molarity solutions.

Answer 1

The Correct Option is A

Step 1: The pH of an acid solution is determined by the hydrogen ion concentration: \[ \text{pH} = -\log [H^+] \] Adding a more concentrated solution of HCl increases the hydrogen ion concentration, lowering the pH.
Step 2: Comparing given solutions: - Increasing the molarity from 0.01 M to 0.02 M results in an increase in \([H^+]\), leading to a lower pH. - Other options either dilute the solution or keep the concentration unchanged, making option (1) correct.