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

Methanoic Acid's dissociation constant

  1. Sep 18, 2004 #1
    I need some help with these, anyone point me in the right direction please?

    The pH of 0.04moldm-3 methanoic acid, HCOOH, is 2.59 at 25 celsius. Calculate the acid dissociation constant at this temperature

    Given that the ionic product of water is 1.0 times 10^-14 mol2dm-6 at 298K, calculate to 3 significant figures the pH at this temperature of a 0.0500M solution of sodium hydroxide.
     
  2. jcsd
  3. Sep 18, 2004 #2

    chem_tr

    User Avatar
    Science Advisor
    Gold Member

    Calculation of acid dissociation constant

    Hello,

    0,04 M of formic acid has pH 2,59, as you gave us.

    HCOOH (0,4-x) ---> H+ (x) + HCOO- (x)

    We should carefully think of this problem first by considering the additional x in (0,4-x); and by omitting the x also. Note that we know x, 10-2,59. It means that the dissociation is about one hundredth; if it would be around 10%, then we should consider it. It won't be any problem if we omit the x in (0,4-x), I hope we've settled this.

    Now the equilibrium constant gives us the dissociation constant (sorry for not even hearing Latex before :smile:):

    K=(10-2,59*2)/0,4=1,652*10-5. The pKa will be found as 4,78.

    About second question, you gave us 0,0500 M of NaOH and want pH as x.xxx. So, the contribution of water may be important. Again, if it is beyond 10%, it can be omitted.

    NaOH (5,000*10-2) --> Na+ (5,000*10-2) + OH- (5,000*10-2) as it is fully ionized.

    H2O --> H+ (1,000*10-7) + OH- (1,000*10-7), ionic product allows me to write these. As you see that they are well beyond 5,000*10-2, it can be omitted, since there is a five hundred thousand-fold difference.

    The overall pH is (with enough precision) found to be 12,699. If you want to add water's hydroxide to it, you'd find the same, as 3 significant figures are not affected with this very small addition.

    Regards, chem_tr
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Methanoic Acid's dissociation constant
  1. Strong Acid Dissociation (Replies: 16)

Loading...