#pH+pOH=14# in aqueous solution under standard conditions.
#HClO_4(aq) +H_2O(l) rarr H_3O^+ + ClO_4^(-)#
Now #"perchloric acid"# is thus stoichiometric in #H_3O^+#, #"hydronium ion"#, and since #pH=-log_10[H_3O^+]#, #pH=-log_10(0.272)=0.565#, and...................