Question

How many moles of sodium hydroxide would have to be added to 250 mL of a 0.422 M hypochlorous acid solution, in order to prepare a buffer with a pH of 7.360? moles

Answer 1

At the end of the titration, all the amount of base get neutralized by the acid. Thus, the resulting solution - will act as a buffer containing a weak acid, Hool and its salt with a strong base, NaOH ie Naoci. - pH of buffer =7.360 molarity of hypochlorous acid = 0.422 m volume of hypochlorous acid = 250 ml -.No.of millimoles of hypochlo sous acid = Molasity x volume (mu) 0.422X250 = 105.5 mmol. nitial Let the no. of a millimoles of NaOH ben HOCK it 'NaOH - Naocl + H2O at t=0 105.5 mmol n 0 0 105. 5 mmol 105.5_n at equilibrium o n n and pH = pka + log (salt to where pka of a HOCI is 7.5 .: 71360 = 7.5 +10g 105.5-n log hos-m * -0.14 = -0.14 son Tosisni = 0.72 = 0.72 105.s-n

n = 0.72(105.5-n). n = 75.96 -0.72n 1.72 n = 75.96 .. n = 75.96 - . = 44:16 mmol = 0.044 mole 1:72 Therefore, the no. of mole of Naoh added to hypochlorous acid to make the Boffer of pH 7. 360 is 0.044 mole.