3.10: Molecular Weight
- Determine the molecular weight of a molecule.
Ions in ionic compounds, are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape.
Molecular Weight
The mass of a molecule—the molecular weight (also called the molecular mass)—is simply the sum of the atomic weights of its atoms. As with formula weights, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular weight.
What is the molecular weight of each covalent compound?
- H 2 O
- C 6 H 6
- NO 2
- N 2 O 5
Solution
Use the atomic weights from the Periodic Table.
The molecular formula H 2 O indicates that there are two hydrogen atoms and one oxygen atom in each molecule. Summing the atomic weights of these atoms,
| 2 H: | 2 × 1.01 = | 2.02 amu |
|---|---|---|
| 1 O: | + 16.00 amu | |
| Total: | 18.02 amu |
The molecular weight of H 2 O is 18.02 amu.
| 6 C: | 6 × 12.01 = | 72.06 amu |
|---|---|---|
| 6 H: | 6 × 1.01 = | + 6.06 amu |
| Total: | 78.12 amu |
The molecular weight of C 6 H 6 is 78.12 amu.
| 1 N: | 14.01 amu | |
|---|---|---|
| 2 O: | 2 × 16.00 = | + 32.00 amu |
| Total: | 46.01 amu |
The molecular weight of NO 2 is 46.01 amu.
| 2 N: | 2 × 14.01 = | 28.02 amu |
|---|---|---|
| 5 O: | 5 × 16.00 = | + 80.00 amu |
| Total: | 108.02 amu |
The molecular weight of N 2 O 5 is 108.02 amu.
Note that the two different nitrogen and oxygen compounds in these examples have different molecular masses.
What is the molecular weight of each covalent compound?
- C 2 H 2
- CO
- CO 2
- BF 3
- Answer a:
-
26.04 amu
- Answer b:
-
28.01 amu
- Answer c:
-
44.01 amu
- Answer d:
-
67.81 amu
Key Takeaways
- A molecule has a certain mass, called the molecular weight.