Adding up the atoms

The idea

The molecular mass of a covalent compound is the sum of the atomic masses of all the atoms in its molecule, each atomic mass multiplied by the number of those atoms in the formula.

Weighing a molecule

You know the formula H₂O. The atomic masses are H = 1 u and O = 16 u.

What do you think the mass of one water molecule is?

• 18 u
• 17 u
• 16 u

Add the atomic masses

Choose a molecule. Each atomic mass is multiplied by the number of those atoms.

Sum of the atomic masses

The molecular mass of a covalent compound is found by adding the atomic masses of all the atoms in its molecule. Multiply each atomic mass by the number of those atoms in the formula.

The molecular mass of a covalent compound is the sum of the atomic masses of all the atoms in its molecule, each atomic mass multiplied by the number of those atoms in the formula.

Notes

Molecular mass = the sum of the atomic masses of all atoms in the molecule, each multiplied by the number of those atoms. H₂O = 18 u, CO₂ = 44 u.

Check yourself

Atomic masses: H = 1 u and O = 16 u. What is the molecular mass of water, H₂O, in u?

Answer: 18 u

(1 u × 2) + (16 u × 1) = 18 u.

Atomic masses: C = 12 u and O = 16 u. What is the molecular mass of carbon dioxide, CO₂, in u?

Answer: 44 u

(12 u × 1) + (16 u × 2) = 44 u.

What is the molecular mass of the oxygen molecule, O₂, in u? (O = 16 u)

Answer: 32 u

Two oxygen atoms: 16 u × 2 = 32 u.

Why is the molecular mass of H₂O not 17 u (1 u + 16 u)?

• Oxygen is counted twice. H₂O has one oxygen atom, so 16 u is multiplied by 1.
• Molecular mass is the product of the atomic masses. Molecular mass is a sum, with each atomic mass multiplied by the number of those atoms.
• There are two hydrogen atoms, so 1 u is multiplied by 2 — correct. Yes!