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Molar Mass - helpMaster

The molar mass of a compound indicates the mass in grams of all the elements in that compound on a per mole basis.

There are four groups of three questions (12 questions in all) in the second activity, titled Calculating Molar Mass. Each question involves the same task. A chemical formula for a compound is given and the student must count the number of atoms of each element and then calculate the molar mass. An example question is shown below.
 

Version 1:

Use the structure of the following table to determine the molar mass of the compound Ba3(PO4)2.

Help for Calculating Molar Mass Activity

There are three tasks to complete in this question:
  • Count the number of atoms of each element shown in the formula.
  • Calculate the mass of the elements using the atomic mass listed for the element in the periodic table.
  • Calculate the molar mass of the compound by adding the masses of each element.
 

Counting the Number of Atoms

Atom Counting was the topic of the first activity. Presumably you learned how to determine the number of atoms of each element in a formula. If not, then review the Help page for the Atom Counting Activity.


 

Determining the Mass of Each Element

Every element has its own unique mass. These mass values are listed in the periodic table, usually to two or more decimal places. They are expressed in units of grams on a per mole basis. Find your most trusted Periodic Table, find the mass of each element and then muliply it by the number of atoms of that element in the formula. The result is that you will know the mass of all the atoms of the particular element. The process must be repeated for each elemet in the compound.


 

Caclulating the Molar Mass

The molar mass of a compound can be determined by considering the contribution of each element to the total mass. Once you have multiplied the number of atoms of each element by the atomic mass of each element (discussed in previous section), you are ready to determine the molar mass. Simply add the individual mass values for each element and you will have the molar mass of the compound.