Cracking the SAT Chemistry Subject Test
CHAPTER 5 CHEMICAL REACTIONS AND
STOICHIOMETRY
Drill 1
- D A diatomic molecule consists of two atoms that are bonded. Among
the choices, only nitrogen monoxide (NO) contains just two atoms. Never
mind that the atoms are from different elements; it’s still a diatomic
molecule. The correct answer is (D).
- E First, eliminate the obvious wrong choices: By inspection, you
know that N 2 O and NO are nowhere near 108 amu, so cross them out. Then
you’ll need to do some math. Use the periodic table to find the atomic
weight of each element in the compounds. Then multiply this mass by the
number of that kind of atom in the molecule. Add the mass contributions
from each element to get the formula weight. When you do this for choice
E, you’ll get 2(14 amu) + 5(16 amu) = 28 amu + 80 amu = 108 amu. (E) is
your answer.
- B Look at the ratios between different types of atoms. If the ratio can
be put in terms of smaller whole numbers, then the formula is not an
empirical formula. Check out choice (B). In this molecule the ratio of
carbon to hydrogen to oxygen is 6:12:6. If we divide this ratio by 6, we’ll
get the simpler (though still equivalent) ratio of 1:2:1. So the empirical
formula of C 6 H 12 O 6 is CH 2 O. Since the molecular and empirical formulas
differ, this must be the answer.
- C Proceed as you did in question 2. If you are comfortable with your
intuitive sense of relative molecular weights, you should be able to
eliminate choices (A) and (E) without doing any calculations. After you get
the formula weight for each choice, compare it to the weight contributed by
oxygen. Look at choice (C). The formula weight of SO 3 is 32 amu + 3(16
