How is a PH of a solution determined?

  1. Hello everyone,

    I'm really confused.

    Here is a equation

    CO2 + H20 <----> H2CO3 <-----> H+ + HCO3-

    * Now if I increase carbon dioxide I know both both Hydrogen and bicarbonate acid increases

    1. Why does this make it more acidic. The thing is what I'm thinking is if a solution has an equal concentration of an acid and base it must be neutral. In this case both hydrogen and bicarbonate have increased in same amounts, shouldn't it be neutral
    2. Why is it that in formula, PH is only determined by H+ concentration, Can't we find PH using OH- concentration.
    3. The definition of base says it must produce OH ions in solution, bicarbonate is not producing OH ions in solution why is it considered a base? Is carbon dioxide a base or acid?
    4. I really don't get what is a weak acid, weak base etc. What is the difference between a weak acid and a base. Isn't a weak acid a base.

    Thanks :)
     
  2. jcsd
  3. Borek

    Staff: Mentor

    Obviously you have a problem with the definition and the difference between base and conjugate base.

    See:
    http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product
    http://www.chembuddy.com/?left=pH-calculation&right=bronsted-lowry-theory

    Neutral means identical concentrations of H+ and OH-.

    pH is defined using concentration of H+, doesn't mean it can't be calculated from known concentration of OH- - but thats just because we know how their concentrations depend on each other.

    Bicarbonate is a Bronsted-Lowry base - it can produce OH- by stealing proton from the water molecule:

    HCO3- + H2O -> H2CO3 + OH-
     
  4. Thanks a lot for your help Borek :smile: It did clear somethings up. I think the water reaction is what confused me. Since water reaction is

    H20 <---> H+ + OH-

    Since this reaction has equal concentration of hydrogen ions and hydroxide ions and it is neutral, I thought if a solution has an equal concentration of acid and base it is neutral. Using the same logic I thought since this reaction H2CO3 <-----> H+ +HCO3- has equal concentration of acid (hydrogen ions) and base (bicarbonate) it would be neutral. But after reading your answer I think even though the concentrations are same bicarbonate is a weak base so it only produces little hydroxide ions so there is more acid (H+) in solution, so the solution is acidic. Hence sulfuric acid is a strong acid.

    So is this concept right

    1. If there are equal concentration of acid and base in a solution, it is not neutral
    but if there are equal concentrations of hydrogen ions and hydroxide ions it is neutral.
    2. K given by the equation 1.7 has a value of 1.8×10-163 (in chem buddy page), where did this value come from? I understood the rest.
    3. 3. What property of bicarbonate makes it a weak base. Is there any way I can work it out, or do I simply have to memorise the weak acids and bases. Simply I'm asking is there a way to work out weak acids and bases, or do I have to memorise them. I think the strong acid and strong bases have H+ and OH- in their name, eg HCL, NaOH. Weak acid and bases, seem to need to react with some other molecule such as water to produce bases and acids.

    Thanks a lot :smile:
     
    Last edited: Feb 14, 2014
  5. Borek

    Staff: Mentor

    Please be specific about what you meanby acid and base - if you refer to a conjugate pair, name them "acid and conjugate base", if you refer to H+ and OH- - don't call them acid and base, call them H+ and OH- or hydronium and hydroxide ions. I feel like part of your problems stem from the slightly lousy language.

    That's where you were wrong.

    This reaction produces H+, so the solutions becomes acidic. Presence of the CONJUGATE base doesn't matter.

    Yes, sulfuric acid is a strong acid, but I fail to see connection to the earlier discussion of carbonic acid (which is a weak one).

    Yes.

    It was measured.

    There is only a bunch of strong acids and strong bases and these you have to memorize. All others are weak ones.

    Nope, that's not correct. Perhaps you are confused by the notation

    HA + H2O <-> A- + H3O+

    but it works for both strong and weak acids. H+ is never alone in the water solution, it is always attached to a water molecule. For practical purposes it is just a matter of notation, see the very first paragraph here: http://www.chembuddy.com/?left=pH-calculation&right=pH-definition
     
  6. Thanks again Borek :smile: However I'm still confused with one part.

    Ok if we consider solution with an equal concetration of H+ and OH- ions as neutral.

    What if there is a solution like this

    X<----> H+ + OH-

    Hydrogen ion concentration= 10 -2
    Hydroxide ion concentration= 10 -2


    Then if I use PH equation = -log [10 -2] = I get an acidic PH.

    But in this case both hydrogen ions and hydroxide ions concentration is same, but PH is acidic instead of neutral. How is that possible. I just made up this equation, is it impossible for something like this to exist?

    Does this mean that the definition of neutral is not actually equal concentration of hydrogen ions and hydroxide ions, but simply the PH of water?

    Thanks :smile:
     
    Last edited: Feb 14, 2014
  7. Borek

    Staff: Mentor

    The only way it can work if for X to be water.

    No, you get a neutral pH. I suppose you use a definitions that says neutral water has pH of 7 - that's not entirely true. Table on the water ion product page shows how neutral pH changes with the temperature. Around freezing neutral water pH is 7.5, around boiling 6.1 - and if you heat water further it goes even lower.
     
    1 person likes this.
  8. I understand now. Thanks Borek :smile:
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?

0
Draft saved Draft deleted