I am still new to learning chemistry so I hope I didn't misunderstand the question completely or make any mistakes. Let me know
Example: Ethanoic Acid, Ka = .000017
Concentration of Ethanoic Acid we were given = .0000001
Write the equation first: CH3COOH + H2O ---> H3O+ + CH3COO-
Then plug this into the equilibrium formula. Products / Reactants (not water) = Ka
[H3O+][CH3COO-] / .0000001 = 000017
As the products are in 1 : 1 ratio you can say [H3O+]Squared / .0000001 = 000017
Transpose the equation:
.0000001 * .000017 = [H3O+]Squared
=.0000000000017
since this is squared and we only want the [H3O+] we square root it.
Square root of .0000000000017 = .000001304
pH = -log [H3O+]
pH = -log .000001304
pH = 5.885