Chemistry
posted by Edward on .
An industrial chemist studying bleaching and sterilizing prepares several hypochlorite buffers. Find the pH of the following buffers.
(a) 0.090 M HClO and 0.090 M NaClO
(b) 0.090 M HClO and 0.135 M NaClO
(c) 0.135 M HClO and 0.090 M NaClO
(d) One liter of the solution in part (a) after 0.0050 mol NaOH has been added.
I only need help with (d). I got 8.07, but that was incorrect

Use the HendersonHasselbalch equation.
For A.
pH = pKa + log (base)/(acid)
pH = pKa + log(ClO^)/(HClO)
pH = pKa + log(0.09)/(0.09)
pH = pKa + log 1
pH = pKa. 
Yeah, I got (a). The answer was 7.53. But when I try to solve (d), I keep getting the wrong answer. I subtracted the moles of NaOH from the acid and added the moles to the base. Then I did Ka = (x*([NaClO]+x))/([HClO  x) and then I found the pH. I got 8.07, but it was wrong.

Is the pKa you're using 7.53? If not be sure and correct the following for your value of pKa. If that's the right answer 7.53 should be pKa.
...........HClO + OH^ ==> ClO^ + H2O
I..........0.09....0........0.09
add..............0.005.............
C........0.005.0.005....+0.005
E..........0.085...0........0.095
pH = pKa + log (base)/(acid)
If I didn't make a math error (and pKa = 7.53), pH = 7.58 
I have no idea why I was having such a hard time with that one. Thank you so much for your help.