weak acid - Strong base titration

Question

anonymous · Answer

Boxed the area where i needed help with. Not sure why but I am very confused with how to find the new concentration of [OH-] & [HA]
what i have done so far is calculated the number of moles of acid and base.
0.025L * 0.1M = 0.0025 moles NaOH added
[A-] = 0.0025 moles base
[HA] = 0.004 - 0.0025 = 0.0015 moles acid

little confused

anonymous · Answer

attachment

vincent-lyon.fr · Answer

I do not understand either.

Steps 1 and 2 are not necessary. imo working out [OH-] as stated is nonsensical.

Go to step 3 first, and everything will be clear. The species present will allow you to calculate the pH immediately.