The amount of pure HCl gas, in mol, that must be added to the solution to lower the pH from 12.0 to 2.0 would be A - VCE - SSCE Chemistry - Question 16 - 2007 - Paper 1

For pH 2.0, the concentration of H⁺ ions is:

$[H^+] = 10^{-2.0} = 0.01 \text{ mol/L}$

The change in concentration of H⁺ ions needed:

$\Delta [H^+] = [H^+]_{final} - [H^+]_{initial} = 0.01 - 1.0 \times 10^{-12} \approx 0.01 \text{ mol/L}$

Assuming we are adding HCl to 1 liter of solution, the amount needed will be equal to the change in concentration:

$\text{Amount of HCl needed} = 0.01 ext{ moles}$

Hence, the amount of pure HCl gas that must be added is approximately 0.01 mol, so the answer is D. 0.01.