The pH of a 0.1 molar solution of the acid HQ is 3. The value of the ionization constant, Ka of the acid is

HQ=H+ + Q-H+  = KaC by Ostwald's dilution lawH+  = 10-pH= 10-3  M;  C = 0.1 MThus,            10-3= Ka× 0.110-6= Ka× 0.1∴    Ka =   10-5