Question 6.
(a) Beaker B has the lowest pH. The NH 4 +ion is the conjugate acid of a weak base, and as an acid (weak)
it will lower the pH.
You get 1 point for the correct beaker. You get 1 point for the explanation.
(b) Solutions A and B produce a gas with a characteristic odor when mixed. The reaction is:
NH 4 +(aq) +OH–(aq) →NH 3 (g) +H 2 O(1)
or
(NH 4 ) 2 SO 4 (aq) +Ba(OH) 2 (aq) →2 NH 3 (g) +2 H 2 O(1) +BaSO 4 (s)
You get 1 point for the correct two solutions, and you get 1 point for either of the balanced equations.
(c) Solution C has the lowest freezing-point depression. This solution is the only nonelectrolyte present.
The electrolytes depress the freezing point to a greater extent.
You get 1 point for the correct solution, and 1 point for the explanation.
(d) Solutions A and D. You get 1 point for each of these you get correct.
(e) Solution C will give no precipitate. Beaker B gives BaSO 4. Beaker D gives Ba 3 (PO 4 ) 2. Beaker E gives
Fe(OH) 2 and BaSO 4.
You get 1 point for choosing solution C. You get 1 point for listing the other precipitates or explaining
that beaker C contains a nonelectrolyte that will not react with either barium ions or hydroxide ions.
Total your points for the different parts. There are 10 possible points.
TOTAL SCORE:
Question 1 ————
Question 2 ————
Question 3 ————
Question 4 ————
Question 5 ————
Question 6 ————
AP Chemistry Practice Exam 2 343