how do I determine the Ka for an acid HX if th epH of a 1.00M solution of HX is 4.5

pH = -log(H^+).

Substitute and solve for (H^+)

......HX ==> H^+ + X^-
I.....1.0M....0....0
C......-x.....x.....x
E....1.0-x.....x....x

Ka = (H^+)(X^-)/HX
x= (H^+) from the pH part. Substitute and solve for Ka.