Ap Chemistry Acid-base 1970-2000
Here is the acid base question and answers for past ap chem tests (link leads to full packet).
1978 A
A 0.682 gram sample of an unknown weak monoprotic organic acid, HA was dissolved in sufficient water to make 50 milliliters of solution and was titrated with a 0.135 molar NaOH solution. After the addition of 10.6 milliliters of base, a pH of 5.65 was recorded. The equivalence point (end point) was reached after the addition of 27.4 milliliters of the 0.135 molar NaOH.
(a) Calculate the number of moles of acid in the original sample.
(b) Calculate the molecular weight of the acid HA.
(c) Calculate the number of moles of unreacted HA remaining in solution when the pH was 5.65.
(d) Calculate the [H3O+] at pH = 5.65
(e) Calculate the value of the ionization constant, Ka, of the acid HA.
Answer:
(a) at equivalence point, moles HA = moles NaOH
= MbVb = (0.0274 L)(0.135 M) = 3.70×10-3 mol H
1984 C
Discuss the roles of indicators in the titration of acids and bases. Explain the basis of their operation and the factors to be considered in selecting an appropriate indicator for a particular titration.
Answer:
An indicator signals the end point of a titration by changing color.
An indicator is a weak acid or weak base where the acid form and basic form of the indicators are of different colors.
An indicator changes color when the pH of the solution equals the pKa of the indicator. In selecting an indicator, the pH at which the indicator changes color should be equal to (or bracket) the pH of the solution at the equivalence point.
For example, when a strong acid is titrated with a strong base, the pH at the equivalence point is 7, so we would choose an indicator that changes color at a pH = 7. {Many other examples possible.}
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