Substance X (MW=356gm) is a weak acid. Its Ka is 1.7x10^-3. Calculate pH of725ml of the following solutions:

Question

a)Containing 0.187 mol of substance x
b)containing 127gm of Substance x

I have been trying to figure this problem out for hours now and do not get it. Please help me and be very detailed. Thank you.

Answer 1

Call the weak acid HA.
(HA) = mols/L = 0.187/0.725 = about 0.26M but you should be more accurate than that.
.......HA --> H^+ + A^-
I.....0.26M....0.....0
C.....-x.......x.....x
E....-0.26-x...x.....x

Ka = 1.7E-3 = (H^+)(A^-)/(HA)
Substitute into Ka expression and solve for x = (H^+) then convert to pH.

Part B is done the same way EXCEPT you don't have mols to start. You convert 127g to mols by mols = grams/molar mass, then M = mols/L solution and from there do the same process as above.
Post your work if you get stuck.

Answer 2

When I solve for H^+ I put 1.7 x10^-3 x 0.26. I got 4.42 x 10^-4. My pH is 3.35. Is that right?