The oxidation number of Al is 3+ while Cr 2 O 7 must be −2.
Therefore, 2Cr must be +12 to result in a net charge of −2.
2Cr = +12; Cr = +6
116 . T, T, CE
An increase in entropy describes an increase in randomness; gas > liquid > solid. When a solid is
dissolved in water, it dissociates and entropy increases as it becomes less ordered.
117. Part C
24 . C
A Lewis base is a substance that can donate a pair of electrons. Among the answer choices,
only PH 3 has a pair of nonbonding electrons, making choice C the correct response. The
easiest way to determine if a substance is a Lewis base is to draw its Lewis dot structure and
see if a lone pair of electrons exists.
25 . E
The difference between SO 2 and SO 3 is 2,300 cal. This quantity must be a negative, since this is
an exothermic reaction.
26 . D
While this is a relatively simple calculation, the question tests your ability to read carefully. The
molecular weight of sulfur hexafluoride is 146 grams/mole. If you divide the mass given, 36.5
grams, by the molecular weight, you find that you have 0.25 mole of sulfur hexafluoride
molecules. A mole is Avogadro’s number of particles; in other words, there are 6.022 × 1023
particles per mole. If you multiply the 0.25 mole of sulfur hexafluoride by Avogadro’s number,
you’ll find the number of molecules of sulfur hexafluoride, 1.511 × 1023 . 1.51 × 1023 is the
number of MOLECULES of sulfur hexafluoride in the sample, not the number of atoms. There
are seven atoms in each molecule, so you must multiply the number of molecules by the
number of atoms per molecule, seven. The trick is in making sure all your units cancel. The
answer is 1.06 × 10^24 atoms.
