PH at the equivalence point?

How can I find the Ka of an unknown acid if the pH at the equivalence point is given?

• I've been trying to figure this problem out for about 3 days now and I still can't get the right answer. My teacher gave us the answer when she saw we were having trouble with it. The answer is 1.6×10^-4 but every signal time I get a different answer. So the problem states a 2.06g solid sample of an unknown monoprotic acid was dissolved in distilled water to produce a 35.0mL solution at 25°C. This solution was then titrated with 0.300M NaOH. The equivalence point was reached when 48.73mL of 0.300M NaOH was delivered. a) Find the number of moles of acid in the solid sample. I found that there were 0.01462 moles of the acid. b) Find the molar mass of the unknown acid. I found it to be 140.9g c) Find Ka for the unknown acid if the pH at the equivalence point is 8.53. I'm not sure what I'm doing wrong. I would really appreciate it if you helped me. Thanks in advanced.

• Answer:

  I'm not going to go into specifics (because I struggle with Chemistry myself and all those numbers are freaking me out) but basically the pH at the equivalence point equals the -log of the Ka. The Ka in that case will be 10^-pH at equivalence point. I hope that helped!