# chem 12

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a) Calculate [H3O^+] in a 0.15M solution of benzonic acid, HC7H5O2(aq) having K of 6.4x10^-5.

b) Calculate the per cent ionization of the HC7H5O2.

I got..

Kb = [HC7H5O3][OH]/HC7H5O2

6.3x10^-5 x Kb =1.0x10^-14
Kb = 1.0x10^-14/6.3x10^-4 = 1.5x10^-10

1.5x10^-10 = (x)(x)/0.15

i'm not sure where i got from there.

• chem 12 - ,

Where i go from there **

• chem 12 - ,

You start over. Benzoic acid is an acid, not a base. Let's call benzoic acid HB.
..........HB ==> H^+ + B^-
init.....0.15.....0.....0
change....-x......x.....x
equil....0.15-x...x.....x

Ka + (H^+)(B^-)/(HB)
Substitute the equilibrium line above into the Ka expression and solve for x = (H^+) = (H3O^+).

b. %ion = [(H^+)/0.15]*100 = ?

• chem 12 - ,

Okay so i got
Ka = (x)(x)/0.15 = 6.5x10^-5

• chem 12 - ,

OK. Ka = 6.5E-5 = (x*x)/0.15; now solve for x which is (H^+) (H3O^+ to be more exact) which is what the question asked. Then use the x value to solve for percent ionization as I showed you in part b.

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