My book asks me to show calculations proving that pKa of H3O+ is -1.74
the way i tried to go about this
H30+ + H20 --> H3O+ + H20
[H3O+] [H2O]
-------------
[H3O+]
since H3O+ is equal to H2O in the numerator, the the Ka of water equals the Ka of H2O(55.5)
thus
-Log(55.5 Ka) = -1.74 = pKa
is my logic correct here?