AP Chemistry Question of the Day --7

Here comes the answer to question 6 I posted yesterday. The correct answer is B. 

As we discussed yesterday, HA is a weak acid and the net ionic reaction is:

Point U is after the equivalent point. At this point, all the HA has been neutralized and the mixture should also contain excessive amount of NaOH. As a result, the dominant species other than the solvent water should be NaA and NaOH. SInce both NaA and NaOH are ionic compounds, they dissociate completely in water. In addition, Na ion and Anion A, and Na ion and hydroxide should exist in 1:1 ratios.  Thus, B is the right answer. 

One thing I need to point out is that even at point U, there should still be a small amount of HA in the solution. This is due to the hydrolysis of the anion A- as shown in the following equation.

Following is Question 7 of the series, it is the continuation of question number 5 and 6. 

7. A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a pH meter and graphed as a function of the volume of 0.100 M NaOH added. At which point on the titration curve is [A–] closest to twice that of [HA]?

A. R

B. S

C. T

D. U

I will post the answer and explanation tomorrow.

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