Which has higher pH: 0.1M boric acid or 0.001M methanoic acid ?

[chickenpop]

Can't figure out how to do this 

Q. Which has higher pH: 0.1M boric acid or 0.001M methanoic acid ?

[JinXi]

Determine the Concentration of [H+] ions by using the Ka values of boric/methanoic acid from the data book. Then -ve log them.

[chickenpop]

How do we determine the Concentration of [H+] ions?

[andy456]

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

[jasoN-]

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

[chickenpop]

awesome So is it always [H3O+]^2, or does it depend on the equation?

[Stroodle]

awesome So is it always [H3O+]^2, or does it depend on the equation?

It is for the weak acids in the data book.