From atoms to molecules
One hydrogen atom has a mass of about 1.67 × 10⁻²⁷ kg, which is far too small to work with comfortably. That’s why lesson 1.3.5 used relative atomic masses: every atom is compared with 1/12 of the mass of a carbon-12 atom, giving H = 1, O = 16, and so on.
Molecules work the same way. The sum of the relative atomic masses of all the atoms in a formula is its relative formula mass相对分子质量, written .
Common relative atomic masses
| Element | Mass | Element | Mass | Element | Mass | Element | Mass |
|---|---|---|---|---|---|---|---|
| H | 1 | C | 12 | N | 14 | O | 16 |
| Na | 23 | Mg | 24 | Al | 27 | S | 32 |
| Cl | 35.5 | K | 39 | Ca | 40 | Fe | 56 |
| Cu | 64 | Zn | 65 | Ag | 108 | Ba | 137 |
Test questions usually give you the relative atomic masses you need, so you don’t have to memorize them all.
How to calculate it
- Count the atoms of each element. A number outside parentheses multiplies every atom inside them.
- For each element: relative atomic mass × number of atoms.
- Add them all up.
Some crystals contain water of crystallizationwater of crystallizationWater molecules built into a crystal in a fixed ratio. Blue copper(II) sulfate crystals, CuSO₄·5H₂O, hold 5 water molecules for every CuSO₄; heating drives the water off and leaves a white powder. Solids like this are called hydrates; lesson 2.5.5 covers them.; they’re called hydrates, such as copper(II) sulfate pentahydrate, . In the formula, the dot means “combined with,” so add the two parts: 160 + 5 × 18 = 250.
Type any formula into the calculator below (capital letters matter) to see how much each element contributes:
Example 1: Carbon dioxide
Find the relative formula mass of .
- Count the atoms. 1 C and 2 O.
- Multiply. C: 12 × 1 = 12; O: 16 × 2 = 32.
- Add. = 12 + 32 = 44.
Example 2: A formula with parentheses
Find the relative formula mass of .
- Count the atoms. The 2 outside the parentheses multiplies every atom inside: 1 Ca, 2 O, and 2 H.
- Calculate. 40 + (16 + 1) × 2 = 40 + 34 = 74.
Common mistakes
Mistake 1: Adding a unit of grams. Relative formula mass is a ratio, so it has no unit. Don’t write 18 g.
Mistake 2: Multiplying only one atom by the number outside the parentheses. isn’t 40 + 16 + 1 × 2 = 58. It’s 40 + (16 + 1) × 2 = 74.
Mistake 3: Including the coefficient. means 2 water molecules, with a total relative mass of 2 × 18 = 36. The relative formula mass of water is still 18.
Quick check
What is the relative formula mass of ? (H = 1, S = 32, O = 16)
1 × 2 + 32 + 16 × 4 = 98.
What is the relative formula mass of ? (Al = 27, S = 32, O = 16)
27 × 2 + (32 + 16 × 4) × 3 = 54 + 288 = 342.
Summary
- Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula. It has no unit.
- A number outside parentheses multiplies every atom inside. For hydrates, add the two parts.
- A coefficient counts molecules and doesn’t change the relative formula mass.
On to AP (for later)
In AP topic 1.1 you’ll learn about molar mass: the mass of 1 mol of a substance in g/mol has the same number as its relative formula mass, so water is 18 g/mol. AP uses the more precise values from the periodic table (H 1.008, O 16.00), so watch your significant figures.