AP Chemistry Question of the Day --6

Here comes the answer to question 5 I posted yesterday. The correct answer is A. 

This is a tricky question! This is the direction the new AP chemistry curriculum is leading to. You  have to be able to read the graph and understand what is going on. 
First of all, you have to decide whether the acid HA is a strong acid or a weak acid, since it is not stated in the question. How are we going to figure that out? Look at the pH of the equivalent point. Because NaOH is a strong base, the pH of the equivalent point would be 7 if HA is a strong acid and the pH of the equivalent point would be higher than 7 if HA is a weak acid. When you examine the graph, you will see that the equivalent point pH is at around 8. Therefore, HA is a weak acid. 

Now we look at point R on the titration curve, it is at the beginning of the curve and has not reached the half point yet. As a result, it is still mainly undissociated HA molecules dominant in the mixture. 

So, the right answer is A.

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

6. 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. Which of the following is the best particulate representation of the species (other than H2O) that are present in significant concentrations in the solution at point U in the titration?

I will post the answer and explanation tomorrow.

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