A) pH of strong acid (HClO4): pH = -log[H+] = -log(2.00*10^-2) = 1.699

B) pH of weak acid(HClO2): pH = 1/2(pKb-log(C)) =
                          
                              = 1/2(1.96-log(0.120)) = 1.44

c) pOH of Sr(OH)2(strong base): pOH = -log(OH-) = -log(2*1.35*10^-2) = 1.569

   pH of the solution =14 -pOH = 14-1.569 = 12.431

D) pH salt of strong base and weak acid (KCN) = 7+ 1/2(pKa +log(c))

                                              = 7+1/2(-log(Ka)+log(C))

                                               = 7+1/2(-log(4.0*10^-10)+log(8.60*10^-2)

                                               = 11.166

E) pH salt of WEAK base and STRONG acid (NH4Cl) = 7- 1/2(pKb +log(c))

                                              = 7-1/2(-log(Kb)+log(C))

                                               = 7-1/2(-log(1.8*10^-5)+log(0.150))

                                               = 5.03