[kaynable]

Any help on how I would figure this out would be great! (It's regarding some Geology well work)

So: there's 15,000 L HCL with a 30% concentration, which is a 9.45M and a pH of -1

Then we add H2O with a pH of 7.4 (which has a 0.000000298 M of HCL).

For the entire solution to change to a pH of 6.4, how much H2O was added???

[Borek]

pH of 0.000000298 HCl is below 7, definitely not 7.4.

Basically this is a simple mass balance - assume x L was added, calculate number of moles in the final solution, same number of moles have been introduced into solution - and it is sum of moles in both initial solutions. That gives one equation and one unknown. But as the data is incorrect, don't expect correct results.

[kaynable]

Whoops, I misunderstood that M part, that can be ignored.  Thanks though, got it figured out now!