Moles = Molarity * Volume
For HCl, Moles = 60 x 10^-3 * 1.2 x 10^-3 = 7.2 x 10^-5 moles
For NaOH Moles = 200 x 10^-3 * 5 x 10^-4 = 10 x 10^-5 moles
NaOh + HCl -> NaCl + H2O
7.2 x 10^-5 moles HCl reacts with 7.2 x 10^-5 moles NaOH to form NaCl and H2O
After reaction, Moles of NaOH left = 10 x 10^-5 – 7.2 x 10 ^-5 = 2.8 x 10^-5 moles
Now volume = 260 mL = 200 + 60. So, [OH-] = 2.8 x 10^-5 / 260 x 10^-3 = 0.010769 x 10^-2 = 1.0769 x 10^-4
pOH = 3.679
