2 moles of CO. Since the stoichiometric ratio of Fe 2 O 3 to CO is 1:3
(based on coefficients from the balanced equation), we see that CO is
limiting. (We would need more than 9 moles of CO based on the 1:3 ratio
for Fe 2 O 3 to be limiting in this case.) The ratio of CO to Fe is 3:2. So 2
moles of CO will produce about 1.3 moles of Fe. You could quickly
estimate this as between 1 and 2 to save time and still get the answer.
- E Consider what gases are being collected in the beaker. Oxygen gas is
flowing in from the reaction. Water vapor, H 2 O(g), is also entering the
beaker from the evaporation of water. The pressure exerted by H 2 O(g) is
equal to the vapor pressure of water. The total gas pressure in the beaker
is, therefore, the sum of the pressure of oxygen gas collected and vapor
pressure of water (at 25°C in this particular problem).
- A If you draw the structure of acetylene, C 2 H 2 , you’ll see that the carbon
atoms must share three pairs of electrons to achieve stable octets.
H – C ≡ C – H
So C 2 H 2 has a triple bond. None of the other molecules
contains a triple bond. Since this is the strongest type of
carbon-to-carbon bond, (A) is correct.
- D Equilibrium is attained when the concentrations of all species become
constant. The concentrations of N 2 O 4 and NO 2 stay the same from the
20-minute mark to the 25-minute mark. This means equilibrium was
achieved before the 20-minute mark. Since the concentrations of N 2 O 4
and NO 2 are different from the 15-minute mark to the 20-minute mark,
equilibrium was not achieved at exactly 15 minutes from the start of the
reaction; equilibrium was attained between 15 and 20 minutes after the
start of the reaction.
