Atomic & Molecular Mass

• Atomic mass unit (u) — 1/12 mass of one carbon-12 atom.
• Atomic mass — relative mass of an atom (e.g. H ≈ 1 u, O ≈ 16 u).
• Molecular mass — sum of atomic masses in a molecule (H₂O = 2×1 + 16 = 18 u).
• Formula unit mass — for ionic compounds (NaCl = 23 + 35.5 = 58.5 u).

1. 1 u = 1.66 × 10⁻²⁷ kg (definition from C-12 standard).
2. Atomic mass — average for naturally occurring isotopes (intro level: use table values).
3. Molecule — group of atoms bonded (O₂, CO₂, NH₃).
4. Calculation — CO₂: 12 + 2×16 = 44 u.
5. Ions in crystals — NaCl formula unit, not discrete molecules.
6. NCERT Ch. 3 — periodic table atomic masses.
7. Real world — molar mass links to grams in mole concept next topic.
8. Precision — use values given in NCERT (Cl ≈ 35.5 u).

Finding molecular mass of sulphuric acid H₂SO₄

Step 1 — Atoms: 2 H, 1 S, 4 O
Step 2 — Mass = 2(1) + 32 + 4(16) = 2 + 32 + 64
Step 3 — = 98 u
Step 4 — Formula unit mass of CaCl₂ = 40 + 2×35.5 = 111 u
Conclusion: add atomic masses from formula.

• Confusing atomic number with atomic mass.
• Misconception: molecular mass has unit grams at this stage (use u).
• Forgetting to multiply by subscript (2 H in H₂O).
• Using molecular mass for NaCl incorrectly (use formula unit mass).
• Rounding 35.5 to 35 for chlorine inconsistently.

• Define atomic mass unit.
• Calculate molecular mass of CO₂ and NH₃.
• What is formula unit mass? Example with NaCl.
• What is the atomic mass of oxygen?

Open the Practice tab for graded questions on Atomic & Molecular Mass.