Stoichiometry: Mass–Mass Problems

In order to work with substances in the laboratory, chemists must work with 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:

If 2.59 g of CaCO₃ was reacted with enough HCl to use up all of the CaCO₃, what mass of HCl would be needed and what mass of CO₂ would be produced?

Use dimensional analysis:

The first factor in the dimensional analysis equation converts the mass of the CaCO₃ to moles by using information concerning molar mass as found from the Periodic Table. The second factor converts moles of CaCO₃ to moles of HCl needed to react with the CaCO₃. 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 CO₂ produced, set up a similar dimensional analysis equation. First convert the mass of CaCO₃ to moles. This time, however, use the mole ratio for CO₂ and CaCO₃ for the second factor. Finally, use the molar mass of CO₂ to convert moles of CO₂ to grams.