10 mL of 10-6 M HCl solution is mixed with 90 mL H2O. pH will change approximately

pH of 10-6 M HCl = 6 on dilution [H+] obtained by using M1V1=M2V2 ;  M1=10-6 M2=[H+]                            V1=10ml   V2=100mlH+ = 10-6 × 10100 = 10-7Thus new pH of HCl is not 7 because it is acid. pH new solution:H+ = 10-7 + 10-7  (from H2O) = 2 × 10-7  ∴   pH = 6.7 (approximately)