Relative molecular mass
Relative molecular mass is the sum of the relative atomic masses of all atoms in a molecule. Like atomic mass, it has no unit.
What it means
A molecule is two or more atoms joined together. Its relative molecular mass (RMM) tells how many times one molecule is heavier than 1/12 of a C-12 atom. In practice we simply add the relative atomic masses of all its atoms. For ionic compounds such as NaCl, the same sum is often called the formula mass.
How to find RMM in four steps
- 1Write the correct molecular formula, for example H₂SO₄.
- 2List each element and count its atoms: H = 2, S = 1, O = 4.
- 3Multiply each count by the element's relative atomic mass: 2 × 1, 1 × 32, 4 × 16.
- 4Add the products: 2 + 32 + 64 = 98. Write no unit (or g mol⁻¹ if the question asks for molar mass).
Worked example: glucose
Find the relative molecular mass of glucose, C₆H₁₂O₆ (C = 12, H = 1, O = 16).
- Carbon: 6 × 12 = 72
- Hydrogen: 12 × 1 = 12
- Oxygen: 6 × 16 = 96
- Add: 72 + 12 + 96 = 180
RMM of glucose = 180.
Ready reckoner (H = 1, C = 12, N = 14, O = 16, Na = 23, Ca = 40, Cl = 35.5)
| Substance | Formula | Working | RMM |
|---|---|---|---|
| Water | H₂O | 2(1) + 16 | 18 |
| Ammonia | NH₃ | 14 + 3(1) | 17 |
| Carbon dioxide | CO₂ | 12 + 2(16) | 44 |
| Ethanol | C₂H₅OH | 2(12) + 6(1) + 16 | 46 |
| Common salt | NaCl | 23 + 35.5 | 58.5 |
| Urea | CO(NH₂)₂ | 12 + 16 + 2(14) + 4(1) | 60 |
| Calcium carbonate | CaCO₃ | 40 + 12 + 3(16) | 100 |
ORS (oral rehydration salts) recipes give the glucose and salt in moles per litre. Makers turn these into grams using molecular masses such as glucose = 180 and NaCl = 58.5.
2 marks: “Calculate the relative molecular mass of nitric acid, HNO₃ (H = 1, N = 14, O = 16).” 1 + 14 + 3(16) = 1 + 14 + 48 = 63.
4 marks: “Define relative molecular mass. Calculate the RMM of glucose (C₆H₁₂O₆) and calcium carbonate (CaCO₃).” Definition with C-12. Glucose: 72 + 12 + 96 = 180. CaCO₃: 40 + 12 + 48 = 100.
Count, multiply, add: count each atom, multiply by its atomic mass, add them all.