Feedback: Multiple Choice Quiz 4

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  Incorrect answers have the "red-lit" button next to your choice.

1. The pKa is the midpoint of the titration, i.e. where [HA] = [A-].

2. The math is: 100 ml*0.05 M = 5*10-3 mol. (Also expressed as 5.0 mmol.)
For a total of 10 additions NaOH, we have
5.0 mmol/10 = 0.5 mmol/addition.
And 1.0 M*0.5 mmol/addition = 0.5 ml.

3. The ratio of 4 can be plugged directly into the Henderson-Hasselbalch equation. But try to think about it this way:
Since there is more A- than AH, the pH will be above the pKa.
How much above? By the log of the ratio (log4 = 2*log2 = 0.6). Therefore, add 0.6 to 7.0 for the correct answer.

4. The various solution forms of phosphate (anions) are listed in Table 2.6. (And the pKa for phosphate is listed as 7.2, not 7.0 as used here.)

5. The phosphate salt requires a cation; the other species show up in a balanced equation for the titration.