quantities they can easily measure. A mole is an amount impractical to actually
count out in the lab. That’s why the concept of molar mass, which relates moles to
mass, was discussed in a previous section of this chapter. Using molar mass
allows problems to be based on mass, which is a measurable quantity. Mass-
mass problems often involve determining the masses of other substances needed
to react with a given mass of a substance or the mass of other substances that can
be produced from that given mass.
Example
Consider the following balanced chemical reaction equation:
CaCO 3 (s) + 2HCl(aq) → CaCl 2 (aq) + H 2 O(l) + CO 2 (g)If 2.59 g of CaCO 3 was reacted with enough HCl to use up all of the CaCO 3 ,what mass of HCl would be needed and what mass of CO 2 would be produced?
Use dimensional analysis:The first factor in the dimensional analysis equation converts the mass of the
CaCO 3 to moles by using information concerning molar mass as found from the
Periodic Table. The second factor converts moles of CaCO 3 to moles of HCl
needed to react with the CaCO 3. The mole ratio was found from the coefficients
in front of each substance in the balanced reaction. The third factor converts
moles of HCl to grams of HCl, once again using molar mass as found from the
Periodic Table.
To find the mass of CO 2 produced, set up a similar dimensional analysis
equation. First convert the mass of CaCO 3 to moles. This time, however, use the
mole ratio for CO 2 and CaCO 3 for the second factor. Finally, use the molar mass
of CO 2 to convert moles of CO 2 to grams.
STOICHIOMETRY: VOLUME-VOLUME PROBLEMS
In the lab, gases are often used and their volumes are easily measured. Recall that
the molar volume of any gas at STP is 22.4 L. So the volume of a gas at standard
temperature and pressure can be converted to moles of that gas via dimensional
analysis. Combining those conversions with the mole ratios found in balanced