Chemistry Reference
In-Depth Information
skip Steps 3 and 5 when we do volume-volume problems! That means that
the procedure for this type of problem becomes:
1. Begin with a balanced chemical reaction.
2. Carefully label the equation with the information from the
word problem.
3. Use the molar ratio shown by the coefficients to determine
the volume of the unknown.
Now let's try the same problem with the shortcut.
2H
2
+ O
2
2H
2
O
hydrogen + oxygen
water
x dm
3
30.0 dm
3
hydrogen
oxygen
coefficients
2
x
1
30.0 dm
3
=
# of moles
Cross-multiplying, we get a volume of 60.0 dm
3
for the hydrogen.
The same answer for less than half the work!
Let's try a couple more problems, using the shortcut.
Example 2
How many dm
3
of oxygen are required to react completely with 5.0
dm
3
of propane when both gases are at STP, according to the balanced
chemical reaction shown here?
5O
2
+ C
3
H
8
4H
2
O + 3CO
2
oxygen + propane
water + carbon dioxide
x dm
3
5.0 dm
3
oxygen propane
coefficients
5
x
1
5.0 dm
3
=
# of moles
Cross-multiplying, we find that the volume of oxygen is 25 dm
3
Example 3
How many dm
3
of methane would react completely with 40 dm
3
of
oxygen gas when both substances are at STP, according to the balanced
chemical equation shown here?
