PH Problem...?????

Question

Calculate the pH of each of the solutions and the change in pH to 0.01 pH units caused by adding 10.0 mL of 2.73-M HCl to 550. mL of each of the following solutions.

Change is defined as final minus initial, so if the pH drops upon mixing the change is negative.

a) water

pH before mixing=7
pH after mixing= ?

The pH after adding an acid I know is going to be <7. I am just having trouble calculating it.

Answer 1

For illustration purpose, we may assume there is a system state after mixing but before reaction.
In this state, [HCl] = 10.0*2.73M /(10.0+550) = 0.04875M.
HCl is s strong acid thus completely dissociate to H+ and Cl- in water. Hence [H+] = 0.04875M.
The contribution to [H+] from water self-dissociation is no more than 10^-7, particularly when there is extra H+ source, and 10^-7 is much less than 0.04875M.
Therefore, pH = -log([H+]) = = -log(0.04875) = 1.31