Answers:
Question 1:
| a) | CH3COOH - it is a
weak monoprotic acid with Ka = 1.8 x 10-5
|
| b) | Halfway to the equivalence
point, exactly half of the original acid has been neutralized and
converted to the conjugate base. Therefore, [CH3COOH] = [CH3COO-].
From the Henderson-Hasselbalch Equation, it is clear that under these
conditions, pH is equal to pKa. Therefore, use the graph to estimate
the volume of NaOH needed to reach the equivalence point. Use the
graph to estimate the pH when HALF of this volume of NaOH has been
added. This pH will be an estimate of pKa.
|
| c) | The pKa for Acetic Acid appears
to be approximately 4.75 (when 10 mL of NaOH have been added, the pH is
approximately 4.75).
|
| d) | The acetic acid has a
concentration of 0.20 M
|
| e) | The pH at the equivalence point
appears to be close to 9.0.
|
| f) | The product of the
neutralization is sodium acetate and water. Sodium acetate
contains the acetate ion, a weak base. Therefore the pH will be
basic at the equivalence point.
|
| g) | Both Phenolphthalein and Phenol
Red change colour at pH values close to that of the equivalence point,
so they would be appropriate choices. Methyl Red changes colour at a
pH that is much too low compared to the equivalence point pH.
|
| h) | The greatest Buffer Capacity
occurs when the acid and its conjugate base are in equal concentrations -
halfway to the equivalence point.
|