Determining Molar Mass Guided Practice – Not Homework – Keep this resource sheet
Now that we know what moles and molar mass are, we are going to work with determining the molar mass for a
compound and also working with moles of that compound. This will work directly with the counting atoms we learned
yesterday
Determining Molar Mass Step 1: Count the number of each element present
For this you will simply count how many of each element is present in the compound. Remember that for parentheses
you multiply all elements inside by the subscript that follows. For example: Al(CO3)2 There is no subscript for Al so that
means we only have 1 Aluminum. There is no subscript for the C, but since there is a subscript 2 for the parentheses
that means 1 x 2 = 2 Carbon. Finally there is a subscript 3 for the O, and a subscript 2 for the parentheses that means 3 x
2 = 6 Oxygen
Determining Molar Mass Step 2: Determine the molar mass for the elements present
For this you will simply look up the atomic mass for each element on the periodic table. Remember the atomic mass is
the decimal point number on the bottom the symbol which is also the molar mass. For example: Al(CO 3)2 We look on
the periodic table and determine the following: Aluminum = 26.98 g/mol Carbon = 12.01 g/mol Oxygen = 16.00 g/mol
Determining Molar Mass Step 3: Multiply the number of atoms present by the molar mass for each element and add
them all together
For this you will simply take the number of atoms present of each element we determined earlier, and multiply it by the
molar mass for that element. You then add these altogether for the total molar mass (also known as the molecular
weight) It will always have the unit g/mol. For example: Al(CO3)2 We determined the following: Aluminum = 1 Carbon
= 2 Oxygen = 6 so we multiply them by their corresponding molar masses. Aluminum = 1 x 26.98 = 26.98 Carbon = 2 x
12.01 = 24.02 Oxygen = 6 x 16.00 = 96.00 We then add it all together. 26.98 + 24.02 + 96.00 = 147.00 g/mol is the
molar mass (formula weight) of Ca(CO3)2
Examples:
NH4NO3
SrCl2
Fe2(SO4)3
Now you try:
CO2
BPO4
(NH4)2S