[skibum143]

a) Choose which of the following is true about the "analyte" (the stuff being titrated) and the "titrant" (the stuff being added) for this titration.

strong base analyte with a weak acid titrant
strong acid analyte with a weak base titrant    
strong base analyte with a strong acid titrant
weak acid analyte with a strong base titrant
strong acid analyte with a strong base titrant
weak base analyte with a weak acid titrant
weak acid analyte with a weak base titrant
weak base analyte with a strong acid titrant

(GRAPH IS ATTACHED)
For this problem, since the pH is decreasing, acid is being added to a base, and since the equilibrium point is = 7 , I thought the answer should be #3: strong base analyte with a strong acid titrant , but it is telling me that is wrong and I have no idea why.

Also: c) If the titrant has a molarity of 0.1250 M and there are 45.00 mL of analyte present, what is the pKa of the analyte in this titration to the nearest 0.5?
I took the equilibrium pH of 7, and did the HH equation: pH = pka + log (base/acid)
7 = pKa + (.0556/.0025)
pKa = 5.65 but it says this is wrong too..

[Borek]

Question about titration curve is tricky.

You are almost right about the plot. I mean - you are right that it looks like the end point is at pH 7.00 and you are almost right drawing the conclusion that is must be a strong base then. However, during strong/strong titration plot should look quite symmetrical, this one doesn't look this way - so there is mor to it.

Download my BATE pH calculator (link in signature), install it (you will be using free 30 day trial, don't worry about paying). Create a new document, select hydrochloric acid, select ammonia, enter 0.1 for both concentrations, select View/Titration from the menu. See the titration curve f ammonia titrated with the acid? Now, change pKb1 to 2, and start playing with concentration of hydrochloric acid - make it more dilute. Can you find the value for which end point pH is 7 and titration curve looks similar to the one frok question? You may need to further modify pKb1 for that, but you should get the idea.

As for c - do I understand it correctly, that you are asked to read the pKb value from the plot? If so, there is a point on the plot where pKa = pH. Actually that was my starting point when I was trying to guess titration curve parameters.