[Nikolai]

Hello, I'm new to the forums and I have a question I'm partially stuck on. I'm currently using Gammon and Ebbing General Chemistry textbook and there is an entire section at the end of the chapter that contains difficult questions (relative) and no helpful examples as to how to solve them - unless your book came with the "manual."

Anyhow, the question is a three-step question and I figured out the first part but couldn't wrap my head around the second and third. If anyone could possibly help me, it would be deeply appreciated. Some tips would be nice too!

A 0.288 gram sample of an unknown monoprotic organic acid is dissolved in water and titrated with a 0.115 NaOH solution. After the addition of 17.54 mL of base, a pH of 4.92 is recored. The equivalence point is reached when a total of 33.83 mL of NaOH is added.

a) What is the molar mass of the organic acid?

0.03383 L NaOH X .115 mol OH^-/1L X 1 mol HA/1 mol OH^- = 3.89 X 10^-3 mol HA

.288g/3.89 X 10^-3 mol HA = 74.0 g/mol

b) What is the Ka value for the acid? (The answers in the back say it should be 1.3X10^-5)

c) Calculate the pH after 35.00 mL of NaOH have been added.

Thank you

[AWK]

b) Use Henderson Haselbach eqation with moles instead of concentrations.
c) cannot be solved unless you know a volume of water added for dissolution of acid