5 Steps to a 5 AP Chemistry 2019

AP Chemistry Practice Exam 2 ❮ 369

Question 6

The compounds are supplied as solutions; therefore, they must be soluble. This means any argument stat-

ing that one or more of the original substances is not soluble in water is invalid.

(a) The lowest pH will be for the solution containing an acid. 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. There are no other

acids present.

You get 1 point for choosing B with the correct explanation.

(b) Beaker E, because transition metal ions are often colored in solution.

You get 1 point for the correct choice.

(c) Solution D—solution D contains the conjugate base, C 2 O 42 - , of a weak acid. The conjugate bases of

weak acids undergo hydrolysis to produce basic solutions.

You get 1 point for the correct choice.

(d) Solution C contains a nonelectrolyte; therefore, it does not conduct. All the remaining solutions con-

tain electrolytes, which are conductors.

You get 1 point for choosing solution C with the explanation.

Total your points for the different parts. There are 4 possible points.

Question 7

(a) Ag 2 SO 4 (s) ^ 2 Ag+(aq) + SO 42 - (aq)

You get 1 point for this equation.

(b) (i) Since the stoichiometries of SrSO 4 and BaSO 4 are the same, the one with the lower Ksp will precipi-

tate first (BaSO 4 ). However, the problem states that it is necessary to show this with calculations.

Since the stoichiometries are the same, it is possible to use the generic equation and generic mass

action expression with M^2 + = Ba^2 + or Sr^2 +.

MSO 4 (s) ^ 2 M^2 +(aq) + SO 42 - (aq) Ksp = [M^2 +] [SO 42 - ]

Since both metals have the same concentration, it is only necessary to determine which requires the

lower sulfate ion concentration. This is done as:

[SO 42 - ] = (^) +
[M ]
sp
2

K

=

[0.10]

Ksp

For BaSO 4 = 1.1 × 10 -^9 M SO 42 - and for SrSO 4 = 3.2 × 10 -^6 M SO 42 -

You get 1 point for determining that the sulfate ion concentration shows that BaSO 4 will precipi-

tate first.

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