Which of the following solutions will turn red litmus blue? pOH 11.50, 2.3 × 10−4 M [OH−], pH 5.50 or 4.1 × 10−5 M [H3O+] Solution  2.3 × 10−4 M [OH−], tip: pH = 14 - pOH = 14 -(-log(2.3x 10-4) = 14-3.64 = 10.36 > pH = 7. The solution is basic and thus has turn red litmus blue. all the other cases, pH < 7 and thus they are acidic..