Predict pH of KBr solution (aq)

  1. 1. The problem statement, all variables and given/known data
    Predict whether an aqueous solution of KBr will be acidic, basic, or neutral.

    2. Relevant equations
    KBr(s) → K+(aq) + Br-(aq)
    Br-(aq) + H2O(l) ← HBr(aq) + H3O+(aq)

    3. The attempt at a solution
    The solution will be neutral.

    I know that this is the correct answer. I just don't know how they got there.
    What I am thinking is: We first dissociate KBr, which will give us the 1st reaction, then we discard the K+ since it's a salt and doesn't affect the pH, and then we see what the reactants are going to be if we dissociate the other product (Br-) in H2O. Apparently, the products of the second reaction are supposed to tell us what the pH of the KBr solution will be.

    How?
    I know that HBr is a strong acid. I would have thought that the solution is acidic, not neutral. What makes it neutral?

    Thank you,

    Joanna.
     
  2. jcsd
  3. I'm a bit concerned here because (to me) it looks like your second reaction doesn't make sense... it doesn't balance at all.
     
  4. That's the answer we got from the teacher... I don't understand it either.
     
  5. We can try another one:

    Predict whether an aqueous solution of KHCO3 will be acidic, basic, or neutral?

    Reactions:

    KHCO3(s) → K+(aq) + CO3-(aq)
    CO3-(aq) + H2O(l) <=> HCO3(aq) + OH-(aq)

    Solution:
    The solution will be basic.

    Explanation: Is it basic because we have OH- as a product?
     
  6. GCT

    GCT 1,769
    Science Advisor
    Homework Helper

    The solution is going to be basic because KHCO3 has a basic component, HCO3-, HCO3- reacts with water

    HCO3- (aq) + H2O (l) <-->H2CO3 (aq)+ OH- (aq)

    bicarbonate is actually an ampholyte, it can be considered as an acid or base, however it is predominantly basic.

    KBr is neutral, because neither components of KBr is acidic or basic.
     
  7. Oh wait, I think I understand about the KBr. Tell me if my reasoning is wrong:

    We get K+, which is a salt, so neither basic or acidic, and we get Br-, which is the conjugate base of HBr => since HBr is a strong acid, it's conjugate base with be very weak, and so it has no real effect on the pH...
    Would that be correct?

    Thanks!
     
  8. GCT

    GCT 1,769
    Science Advisor
    Homework Helper

    K+ can be considered as an acid/base, but it is indeed weak with respect to both, your argument for the Br- is good.
     
  9. Thank you!
     
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