equations, for reactions involving gases, allows for volume-volume
stoichiometry problems to be calculated. In volume-volume problems, you are
given the volume of one gas at STP and asked to determine the volume(s) of other
gases involved in the reaction.
Example
Consider the following balanced chemical reaction equation:
N 2 (g) + 3H 2 (g) → 2NH 3 (g)To produce 0.400 L of NH 3 at STP, what volumes of nitrogen and hydrogen,also at STP, would be required?
Use dimensional analysis:
The first factor in the dimensional analysis equation converts the volume of
NH 3 to moles. The second factor uses the mole ratio from the balanced equation
to convert moles of NH 3 to moles of N 2. Finally, the third factor converts moles of
N 2 to the volume of N 2. Mathematically, factors #1 and #3 simply undo each
other. They display a relationship that was suggested in Chapter 5 by the Ideal
Gas Law as well as by Avogadro’s Hypothesis (discussed in this chapter).
Namely, there is a direct relationship between the volumes of gases, at the same
temperature and pressure, and their numbers of particles (measured in moles). In
other words, mole ratios can be construed as volume ratios between gases
existing at the same temperature and pressure. The only factor needed to solve the
previous problem mathematically was factor #2. To express the dimensional
analysis equation properly, though, requires using the mole ratios as volume ratios
as is done here:
To find the amount of H 2 required to produce the 0.400 L of NH 3 requires asimilar equation but with a different ratio between the gases:
Using mole ratios, i.e., the coefficients from the balanced equation, as
volume ratios saves time. It also makes volume-volume problems less
cumbersome to solve. The relationship between the volumes of reacting gases