Which of the following could be added to 100 mL of 0.01 mol L⁻¹ hydrochloric acid solution to change its pH to 4? (A) 900 mL of water (B) 900 mL of 0.01 mol L⁻¹ hydrochloric acid (C) 9900 mL of water (D) 9900 mL of 0.01 mol L⁻¹ hydrochloric acid - HSC - SSCE Chemistry - Question 12 - 2016 - Paper 1

At pH 4, the concentration of hydrogen ions [H⁺] is:

$[H^+] = 10^{-4} \, \text{mol L}^{-1}$

Using the dilution equation:

$C_1V_1 = C_2V_2$

where:

• $C_1 = 0.01 \, \text{mol L}^{-1}$ (initial concentration).
• $C_2 = 10^{-4} \, \text{mol L}^{-1}$ (target concentration).
• $V_1 = 100 \, \text{mL}$ (initial volume).

We need to find $V_2$:

1. Rearranging gives:

   $V_2 = \frac{C_1V_1}{C_2} = \frac{0.01 \times 100}{10^{-4}} = 10000 \, \text{mL}$

To achieve a total volume of 10000 mL starting from 100 mL, we need:

$\text{Volume to add} = V_2 - V_1 = 10000 - 100 = 9900 \, \text{mL}$

Therefore, 9900 mL needs to be added.

From the options provided, the correct answer is:

(C) 9900 mL of water