ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: chickenpop on November 10, 2010, 07:26:25 pm
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Can't figure out how to do this :-\
Q. Which has higher pH: 0.1M boric acid or 0.001M methanoic acid ?
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Determine the Concentration of [H+] ions by using the Ka values of boric/methanoic acid from the data book. Then -ve log them.
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How do we determine the Concentration of [H+] ions? :P
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Square root the Ka value in the data book (then multipy by the concentrations??) then cuck it in the log formula...
not sure about the middle part
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Firstly find the Ka values of both species (data booklet)
They are both <10^-4, hence we assume they are weak and ionisation is negligible
Boric acid: H3BO3 + H2O <--> H2BO3- + H3O+
H2BO3- will unlikely ionise due to it's now negative polarity.
Ka = 5.8 x 10^-10 = [H3O+]^2 / [H3BO3] [H2BO3-] = [H3O+] as they are produced in a 1:1 ratio
[H3BO3] = 0.1M (given)
therefore: [H3O+]^2 = 5.8 x 10^-10 x 0.1
[H3O+] = 7.62 x 10^-6
pH = -log10[H3O+] = 5.12
Methanoic Acid: HCOOH + H2O <--> HCOO- + H3O+
Ka = 1.8 x 10^-4 = [H3O+]^2 / [HCOO-] [HCOO-] = [H3O+] as they are produced in a 1:1 ratio
[HCOOH] = 0.001M (given)
therefore: [H3O+]^2 = 1.8 x 10^-4 x 0.001
[H3O+] = 4.24 x 10^-4
pH = -log10[H3O+] = 3.37
Therefore boric acid has a higher pH
hope it is right
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awesome :) So is it always [H3O+]^2, or does it depend on the equation?
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awesome :) So is it always [H3O+]^2, or does it depend on the equation?
It is for the weak acids in the data book.