# Homework Help: PH concentrations, strong acids and water

1. Jan 22, 2010

### nobahar

Hello!
I was wondering, with questions such as "What is the pH of 0.05 mol dm-3 of HCl?", you specify the pH by simply using the concentration, as it completely dissociates. However, I assume that at such a concentration like this it is not 'pure', and would be present in water. Shouldn't I then have to consider the [H+] and [OH-] concentrations of water?
Thanks for any help.

2. Jan 22, 2010

### CompuChip

They are in water, that's what mol/dm3 means.
So in every liter of solution, there is 0.05 mol of HCl. The rest is pure water, which is neutral (in the sense that the concentration of H+ and OH- is equal to great accuracy).

3. Jan 22, 2010

### nobahar

so shouldn't the [H+] introduced be included with the [H+] of the water for an overall pH value?
Sorry, it has not been a good day...

4. Jan 23, 2010

### Staff: Mentor

If concentration of acid is high enough they can be safely ignored. High enough in this context means more than 5*10-7M.

See calculation of pH of strong acid for more details.

Last edited by a moderator: Aug 13, 2013
5. Jan 23, 2010

### nobahar

Ah, many thanks Compuchip and Borek. I looked into calculating pH, and I have covered it in class, I couldn;t find it mentioned on a site, and I don't think it came up in class, although it may have done...
Once again, many thanks!