PLEASE HELP ME WITH CHEM TITRATION PROBLEM??!!!

Question

A 25.0 mL sample of 0.150 M hypochlorous acid is titrated with a 0.150 M NaOH solution. What is the pH after 26.0 mL of base is added? The Ka of hypochlorous acid is 3.0 × 10-8.

 

7.51
11.47
7.54
2.54
7.00

 

I have tried everything and I just don't know what I am doing. I thought I would try to set up an ice chart based on the premise of:

      HA  +             OH- <-->      A- +       H2O

  I   0.00375mol   0.0039 mol      0 mol     0 mol

 C   -0.0039 mol  -0.0039 mol     +0.0039 mol

E = 0.00015 mol   0 mol              0.0039 mol

 

 

But then I get stuck and I know I need the concentration of H or OH and I think this is a great example of how I have no clue what I am doing!! All help is appreciated!! I am feeling awful about this..

Answer 1

Instead of subtracting 0.0039 mol, use 0.00375 mol to calculate 0.00015 mol of -OH.

The strong base would consume the weak acid, so the remaining OH yields pOH = -log [0.00015/(0.026+0.025)] = -log[0.003] = 2.5

Then pH = 14 - pOH = 14-2.5 = 11.5