AP Chemistry Question of the Day --8

Here comes the answer to question 7 I posted yesterday. The correct answer is C. 

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

If HA and A-  ion coexist, the reaction has not reached equivalent point yet. answer D can be eliminated. From the graph, we can see that at equivalent point, 40ml of NaOH solution is consumed. This means S is the half point, when half of the HA is neutralized. point R is before the half point. At point R, HA concentration is higher than A- ion. At point T, there are more A- on than HA. So answer C is the correct answer.

Following is Question 8 of the series, it is the continuation of question number 5, 6 and 7. 

8. 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. A student carries out the same titration but uses an indicator instead of a pH meter. If the indicator changes colour slightly past the equivalence point, what will the student obtain for the calculated concentration of the acid?

A. Slightly less than 0.0800M

B. Slightly more than 0.0800M

C. Slightly less than 0.125M

D. Slightly more than 0.125M

I will post the answer and explanation tomorrow.

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