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Question on pH scales

  1. Dec 27, 2011 #1
    Ok so i have this question to figure out.

    What is the pH of a solution having an H2CO3 concentration of
    10-7 mol l-1?

    Now here is the info i've been given to learn about pH in my lecture notes.

    That pH is the negative log to base 10 of the concentration of hydrogen ions in a solution. It says that for pure water pH=-log10[H+ (aq)]

    For pure water [H+] = 10-7 mol l-1

    Therefore pH of pure water is -log10 10-7=7

    Two questions...

    a) Why does H+ = 10-7 mol l-1 in pure water? What is it about H20 that actually tells me that? How do you work out the concentration of H+ in H20

    b) why do you have an H+ in water when its H20? when I think about water it is balanced by 2H to an O so why are there ions? Or is it saying that the amount of H+ in water is 10-7 mol -1 as an overall figure ? Its just that i thought that the anions in O balanced the cations in H perfectly so why am I being told about the amount of H+ in water? Is this why water has a pH of 7 and is neutral pH?

    As you can see I am quite confused.

    How would I be able to work out the concentration of H+ ions in a solution of H2CO3 10-7 mol l-1?

    I don't really understand the concept and have been quite confused by any online material I have come across. Links to any simple explanations would be appreciated as well as any comments.
  2. jcsd
  3. Dec 27, 2011 #2


    User Avatar

    Staff: Mentor

    First of all, looks like you have never heard about water autodissociation.

    But it is not all you will need to solve the question, water containing 10-7M H2CO3 is not pure. And the way the question is worded it is not clear to me if 10-7M is the equilibrium concentration of H2CO3, or analytical concentration of all forms of carbonic acid.
  4. Dec 28, 2011 #3
    Hi Borek the pure water was just given as an example not as a question. The question was to figure out the pH of H2CO3 to 10-7 mol l-1. I think that its an equilibrium as thats what we have been learning and I would imagine that it dissocociates to H+ and HCO3-. Sorry if I don't sound very scientific with my explanation by the way.

    Assuming that this is correct would that make the pH = 7?

    I am thinking that water is evaporated from the ocean then combines with CO2 making carbonic acid which then rains into rivers and lakes where it then dissociates and erodes the rocks , this is basically what we are learning about. But the thing I really need to understand is the concept of why the H+ in a concentration of 10-7 mol l-1 = pH 7. In other words what would make it a 6 or a 5 or an 8 if it is related to the concentration of H+?. If for example we were told that a concentration of H2CO3 was 10-8 mol l-1 would that by default give it a pH of 8 assuming the same dissociation?
  5. Dec 28, 2011 #4


    User Avatar

    Staff: Mentor

    Still not clear to me what you are asking about. Because of the autodissociation pure water pH is around 7, if you add acid it can't go up to 8 and make water alkaline.

    Compare pH of a strong acid solution.
  6. Dec 28, 2011 #5
    OK well don't know what else to tell you, thats the way the question is worded so will just submit what i think the answer is and hope for the best.

    Many thanks for all your help.
  7. Jan 1, 2012 #6
    Check page 8 in the lecture notes (lecture 10) and solve as shown in example! Easy. Good luck!
  8. Jan 2, 2012 #7
    I was looking at the page on pH about 4 pages back thinking thats all there was, many thanks. Who are you by tyhe way?
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