Cliffs AP Chemistry, 3rd Edition

Solving for x, you obtain 32.00 grams/equiv. for metal M. From this information, it would

seem reasonable that the unknown metal is copper, forming the compound CuS. Copper,

with a +2 valence, has an equivalent weight of 31.78.

55.(C) The law of Dulong and Petit states that

(molar mass) ×(specific heat) ≈ 25 J/mole ⋅°C

Substituting the given information into this relationship yields

(xg/mole) ×0.052 cal/g ⋅°C ≈ 25 J/mole ⋅°C

Because you know that 4.184 joules = 1 calorie, convert the 25 joules to calories so that

units can cancel.

(xg/mole) ×0.052 cal/g⋅°C = 6.4 cal/mole ⋅°C

Solving for xyields ~124. You can also learn something more about the metal from the

concept of equivalent weights.

0.80 gram oxygen/equivalent #8 = 3.00 grams x/equivalent #?

? = 30 is the gram-equivalent weight for the metal. However, this assumes that the charge

of the metal is +1. Because equivalent weight = atomic weight/valence number,

30 gram-equivalent weight = 120 g ⋅mole–1/x

x = +4, which means that the metal, whose atomic weight is approximately 120, has a

valence of +4. Thus, the metal is probably tin.

56.(A)

/

Density massV PMMRT

L atm mole K K

atm g mole

L

g

:^11

:

:::

:

== = -- =

Many students would choose answer choice (E). The reason molecular mass is the wrong

answer is the question did not state the conditions when comparing one gas to another. For

example, 1 mole of hydrogen gas at 50 K in a 1.0-liter container might have a higher den-

sity than 0.01 mole of uranium hexafluoride (UF 6 at 200 K in a 20-liter container, even

though the UF 6 has a greater molecular mass.

57. (D) Element A keys out to be Al, which, being a metal in Group IIIA, would have a +3
    charge. Element B would key out as sulfur, a nonmetal with a charge of −2, giving the for-
    mula Al 2 S 3 , or A 2 B 3.
    58.(A) If you missed this question, go back to the rules for naming coordination compounds
    found in the answer to question 22 (page 357).
    The (III) indicates that the central positive ion should have a +3 charge. In adding up the
    charges for the ligand you get:
    2 H 2 O = 0
    2 NH 3 = 0
    2(Br–) = − 2
    1 (Cr+?) =?
    +1 (overall charge of compound)

Part IV: AP Chemistry Practice Test