Phenol, C6H5OH is a weak organic acid:?

Phenol, C6H5OH is a weak organic acid:

C6H5OH (aq) + H2O (l) = C6H5O- (aq) + H3O+ and Ka = 1.3 x 10-10.

If you dissolve 0.417 g of phenol (molecular mass = 94.0 g/mol) in water with a solution volume of 210 mL, what is the equilibrium hydronium ion concentration? What is the pH and pOH of the solution?

Answer:
Moles phenol = 0.417 / 94 = 0.00444
Initial concentration = 0.00444 / 0.210 L = 0.0211 M
C6H5OH + H2O <----> C6H5O- + H3O+
initial concentration
0.0211
change
- x . . . . . . . . . . . .. . . . . . .+x . . . . . . .. +x
at equilibrium
0.0211- x . . . . .. . . . . . . . . .x. . . . . . . . . .x
1.3 x 10^-10 = x^2 / 0.0211-x
x = 1.66 x 10^-6 M = [H3O+]
pH = - log 1.66 x 10^-6 = 5.78
pOH = 14- pH = 8.22

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