I know these questions are very specific and can be hard to answer with little i
ID: 706380 • Letter: I
Question
I know these questions are very specific and can be hard to answer with little information, but please try.
Part I:
1. What is the pH at the equivalence point of the vinegar solution titration?
2. How many moles of NaOH were added to reach equivalence point of the solution?
3. What is the concentration of the acid in the vinegar assuming it is a monoprotic acid?
Analysis and Discussion:
1. In part I, are the pKa values close to each other? Should they be the same? Explain.
2. Does the phenolphthalein indicator change color at the equivalence point, or at another point?
3. Describe the appearance of the titration curve.
4. Based on the titration curve, what is the pKa value of the acid in the vinegar solution? How did you determine it?
5. Does the pKa value of the vinegar from the titration in part II agree with the values you obtained in Part I? Should they agree? Explain.
Explanation / Answer
Part I:
1. Depending on the concentration, pH at the equivalence point of the vinegar solution titration lies between 8 and 9.
2. Answering how many moles of NaOH were added to reach equivalence point of the solution is impossible without knowing the strength of vinegar solution. A very high concentrated vinegar (say 80%) will require only 0.3 mL when 0.1N strength NaOH is used in a 50mL burette. So the vinegar solution has to be diluted and then the calculation can be done using V1S1=V2S2 where V is the volume and S is the strength.
3. Again, determination of the concentration of the acid in the vinegar unless we know the number of moles of NaOH. Moreover vinegar not only contains acetic acid, but also some other acids in very minute quantities are added to the vinegar solution. The main acid being acetic acid, the reaction goes as CH3COOH + NaOH ? CH3COONa + H2O. As NaOH and CH3COOH, calculation of any one will let us know the other.