How to Find Molecular Mass (Molecular Weight)

The molecular mass or molecular weight is the total mass of a compound. It is equal to the sum of the individual atomic masses of each atom in the molecule. It's easy to find the molecular mass of a compound with these steps:

1. Determine the molecular formula of the molecule.
2. Use the periodic table to determine the atomic mass of each element in the molecule.
3. Multiply each element's atomic mass by the number of atoms of that element in the molecule. This number is represented by the subscript next to the element symbol in the molecular formula.
4. Add these values together for each different atom in the molecule.

The total will be the molecular mass of the compound.

Example of Simple Molecular Mass Calculation

For example, to find the molecular mass of NH₃, the first step is to look up the atomic masses of nitrogen (N) and hydrogen (H).

H = 1.00794
N = 14.0067

Next, multiply the atomic mass of each atom by the number of atoms in the compound. There is one nitrogen atom (no subscript is given for one atom). There are three hydrogen atoms, as indicated by the subscript.

molecular mass = (1 x 14.0067) + (3 x 1.00794)
molecular mass = 14.0067 + 3.02382
molecular mass = 17.0305

Note the calculator will give an answer of 17.03052, but the reported answer contains fewer significant figures because there are six significant digits in the atomic mass values used in the calculation.

Example of Complex Molecular Mass Calculation

Here's a more complicated example: Find the molecular mass (molecular weight) of Ca₃(PO₄)₂.

From the periodic table, the atomic masses of each element are:

Ca = 40.078
P = 30.973761
O = 15.9994

The tricky part is figuring out how many of each atom are present in the compound. There are three calcium atoms, two phosphorus atoms, and eight oxygen atoms. How did you get that? If part of the compound is in parentheses, multiply the subscript immediately following the element symbol by the subscript that closes the parentheses.

molecular mass = (40.078 x 3) + (30.97361 x 2) + (15.9994 x 8)
molecular mass = 120.234 + 61.94722 + 127.9952
molecular mass = 310.17642 (from the calculator)
molecular mass = 310.18

The final answer uses the correct number of significant figures. In this case, it's five digits (from the atomic mass for calcium).

Tips for Success

• Remember, if no subscript is given after an element symbol, it means there is one atom.
• A subscript applies to the atom symbol it follows. Multiply the subscript by the atom's atomic weight.
• Report your answer using the correct number of significant figures. This will be the smallest number of significant figures in the atomic mass values. Watch rules for rounding and truncating, which depend on the situation.