Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: PH concentrations, strong acids and water

  1. Jan 22, 2010 #1
    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. jcsd
  3. Jan 22, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    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).
  4. Jan 22, 2010 #3
    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...
  5. Jan 23, 2010 #4


    User Avatar

    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
  6. Jan 23, 2010 #5
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook