0.0400 M solution of HClO2, pH = 1.80 solve for Ka

Ka for weak acids is - - - > Ka = (X) ² / (initial - X)

initial = 0.0400 M

to find X, you can find the H concentration using pH

[H+] = 10^-pH

[H+] = 10^-1.80 = 0.016 which is X

ka = (0.016) ² / (0.0400 - 0.016) = 0.011