atomic weight is 40 amu, nitrogen’s is 14 amu, and oxygen’s is 16 amu.
That means the formula weight = 40 + 2(14 + 3(16)) = 40 + 2(14 + 48) = 40
+ 28 + 96 = 164 amu. (B) is correct.
- D You’re given a substance’s mass composition, and you need to
determine its empirical formula. So imagine that you have 100 g of the
substance.
• You’ve got 9 g of magnesium, and magnesium’s
atomic weight is 24 amu, so you have = 0.375
mole of magnesium atoms.
• You’ve got 91 g of iodine, and iodine’s atomic
weight is 127 amu, so you have = 0.717 mole
of iodine.
• The ratio of iodine atoms to magnesium atoms is
, which is very close to 2:1.
This means that the empirical formula of the substance
is MgI 2 , and that’s why (D) is correct.
Drill 2
- A You know to associate Gibbs free energy with the spontaneity of
reactions—that alone is enough to tell you which answer choice is correct.
But if you want to take it further, remember that a reaction is spontaneous if
the overall combination of enthalpy change—energy change—and entropy
change is energetically adequate. Even an endothermic reaction—a reaction
