Final answer:
The pH of a 0.0000000005 M aqueous solution of HCl is 9, which implies that the solution is basic due to the very low concentration of HCl and the impact of water's autoprotolysis.
Step-by-step explanation:
The pH of a 0.0000000005 M aqueous solution of HCl is calculated using the formula pH = -log[H3O+]. Since HCl is a strong acid, it dissociates completely in water, so the concentration of hydrogen ions [H+] is equal to the concentration of HCl. Therefore, the pH is calculated as follows:
pH = -log(0.0000000005)
pH = 9
This indicates that the solution is basic because the pH is greater than 7. Normally, a solution of HCl would be expected to be acidic; however, the concentration in this instance is so low that the water's autoprotolysis (where water self-ionizes to form H+ and OH- ions) affects the pH, making the solution slightly basic.