front of H 2 O gives us 4 oxygen atoms on the right. However, now we
cannot balance carbon without upsetting the oxygen balance. So (B) is also
wrong. What about (C)? If the coefficient for O 2 is 3, then we have 6
oxygen atoms on the left. Putting a 2 in front of CO 2 and H 2 O gives 6
oxygen atoms on the right. So far, so good. Notice that putting a “1” in front
of C 2 H 4 puts carbon and hydrogen in balance, so the answer is (C).
- A Notice that the consumption of 2 moles of Na releases 822 kJ of
heat. What happens if only 0.5 mole of Na is consumed? Since 0.5 is only
25% of 2, only 25% of 822 kJ of heat will be released. As you can see,
choice (A) is about one-quarter of 822 kJ, so the correct answer is (A). - B When information about more than one reactant is given, brace
yourself for a limiting reactant question. The stoichiometric mole ratio of
Al to Fe 2 O 3 is 2:1. The atomic weight of 1 aluminum atom is roughly 27
amu, which means that 1 mole of Al has a mass of about 27 g, so 80 g of Al
represents nearly 3 moles. The formula weight of Fe 2 O 3 is 160 g, so we
have approximately 0.5 mole of Fe 2 O 3 . We’ve calculated that the actual
mole ratio of Al to Fe 2 O 3 is roughly 3:0.5 (or 6:1). This means that Fe 2 O 3 is
the limiting reagent. Notice from the balanced equation that for every 1
mole of Fe 2 O 3 consumed, 2 moles of Fe are produced. If 0.5 mole of Fe 2 O 3
is consumed, then 1 mole of Fe is produced. The correct answer is (B).