Molecular and formula unit mass

Molecular mass = the total of the atomic masses of all the atoms in one molecule. Use H = 1, C = 12, N = 14, O = 16 (in u).

• H₂O = 2 × 1 + 16 = 18 u.
• CO₂ = 12 + 2 × 16 = 44 u.
• HNO₃ = 1 + 14 + 3 × 16 = 63 u.
• CH₄ = 12 + 4 × 1 = 16 u.

Ionic compounds are different. They form 3-D crystals, not separate molecules, so we must not call NaCl a molecule. Instead we use the simplest whole-number ratio of its ions, the formula unit, and its mass is the formula unit mass.

• Na₂O = 2 × 23 + 16 = 62 u.
• KCl = 39 + 35.5 = 74.5 u.

Brackets multiply everything inside them:

• Ca(NO₃)₂ = 40 + 2 × (14 + 3 × 16) = 40 + 2 × 62 = 164 u.
• Mg(OH)₂ = 24 + 2 × (16 + 1) = 58 u.

Counting particles in ions. An ion has the same protons as its atom but a different number of electrons:

• Na⁺: 11 protons, 10 electrons (lost one).
• Br⁻: 35 protons, 36 electrons (gained one).
• Hg²⁺: 80 protons, 78 electrons (lost two).