Question

I have another similar problem. I have calculated everything given (Correctly), but I can't seem to get the right pH for the stoichiometric point (I keep getting 7.16)
Question:
Suppose that 50.0 mL of 0.25 M CH3NH2(aq) is titrated with 0.35 M HCl(aq). Refer to table 1 and table 2.

(e) What volume of 0.35 M HCl(aq) is required to reach the stoichiometric point?
35.7 mL (correct answer)

(f) Calculate the pH at the stoichiometric point.

I set up the "PUG" Chart, found my concentrations of the base to be 5.83E-5M and the acid .146M

I then used the expression:
HB^+ +H2O <--> H3O^+ + B to set up the ICE chart. I then ended up with

Ka= (1E-14)/(3.6E-4)=(5.83E-5)x / (.1458)

since I found the change in x to be negligble. 3.6E-4 is the given Kb for the base.

Could anyone shed some light on my calculations?
Thanks!

Answer 1

I think your problem is that you have substituted 5.83E-5 for the base while letting x stand for (H3O^+). x is ALSO (CH3NH2) at that point. The base, at that point, is ONLY the amount from the hydrolysis of the salt. I agree that x in the denominator is small and can be neglected. I think the pH at the equivalence point is 5.69 or close to that. 0.1458 is concn of the salt. By the way, what is a PUG chart. I find most student know what an ICE chart is; perhaps they know a PUG chart, too, but I don't.