Formula mass

The sum of the atomic masses of all the atoms which make up the compound is known as the formula mass of a compound.

Formula mass is a general term applied to the mass of a substance based on summing up the atomic masses of the atoms in the chemical formula regardless of the type of bonding between atoms in the chemical formula.

If the substance has covalent bonding, we can specifically refer to the sum of the atomic masses of all the atoms which make up the compound the molecular mass.

Sometimes, formula mass is also referred to as molar mass.

Example - Determine the formula mass of ammonium sulfate, (NH₄)₂SO₄

Use the periodic table to determine the formula mass of one formula unit of ammonium sulfate. The chemical formula of ammonium sulfate (ie - the formula unit) is (NH₄)₂SO₄.

Solution:

1. 

   Count the number of atoms in (NH₄)₂SO₄
   Figure out how many atoms of each element there are in one formula unit.
2. Use the periodic table to figure out the atomic mass of each element in the formula unit.
3. Multiply the number of atoms (from step 1) of each element by its atomic mass (from Step 2) to obtain the mass of that particular element in one formula unit.
4. Add up the masses of each element (from Step 3) to get the formula mass of the compound.

Types of atoms	Number of atoms (Step 1)	Atomic Mass (Step 2)	Mass of each type of atoms (Step 3)
N	2	14.007 amu	28.014 amu
H	8	1.008 amu	8.064 amu
S	1	32.06 amu	32.06 amu
O	4	15.999 amu	63.996 amu
Formula Mass of (NH4)2SO4 (Step 4)			132.134 amu

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BCIT courses: CHEM 0011