Library / Foundations: Chemistry / FCH-01
Foundations: Chemistry
Relative formula mass
Add up the relative atomic masses of every atom in a formula.
- Ref
- FCH-01
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- Mr
n is the number of atoms of each element in the formula (use a bracket multiplier where needed) and Ar the relative atomic mass from the periodic table. The calculator allows three different elements.
Count each type of atom in the formula, multiply by its atomic mass, and add everything up.
Calculator
- Molar mass40 g/mol
- Share of element 1 by mass57.5 %
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Mr has no unit; the molar mass is the same number in g/mol.
- Everything else in quantitative chemistry (moles, yield, concentration) starts from Mr.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| n₁ Atoms of element 1 | 1 to 12 | 40 | 293 | 253 | +10% | +5.75 % |
| Ar₁ Atomic mass of element 1 | 1 to 240 | 18 | 257 | 239 | +10% | +5.75 % |
| Ar₂ Atomic mass of element 2 | 1 to 240 | 25 | 264 | 239 | +10% | +4 % |
| Ar₃ Atomic mass of element 3 | 1 to 240 | 40 | 279 | 239 | +10% | +0.25 % |
| n₂ Atoms of element 2 | 1 to 12 | 40 | 216 | 176 | +10% | +4 % |
| n₃ Atoms of element 3 | 1 to 12 | 40 | 51 | 11 | +10% | +0.25 % |
Sweep of Atomic mass of element 1
| Atomic mass of element 1 | Relative formula mass |
|---|---|
| 1 | 18 |
| 30.88 | 47.88 |
| 60.75 | 77.75 |
| 90.63 | 107.6 |
| 120.5 | 137.5 |
| 150.4 | 167.4 |
| 180.3 | 197.3 |
| 210.1 | 227.1 |
| 240 | 257 |
Sweep one variable
Variables and how each one contributes
Relative formula mass
What the equation gives you. Everything else feeds it.
Atoms of element 1
How many atoms of the first element are in the formula. Adds n₁ × Ar₁ to the total.
Atomic mass of element 1
Relative atomic mass of element 1 (Na = 23). Raises the total in proportion to n₁.
Atoms of element 2
How many atoms of the second element. Adds n₂ × Ar₂.
Atomic mass of element 2
Relative atomic mass of element 2 (O = 16). Raises the total in proportion to n₂.
Atoms of element 3
How many atoms of the third element. Adds n₃ × Ar₃.
Atomic mass of element 3
Relative atomic mass of element 3 (H = 1). Raises the total in proportion to n₃.
Worked example with the values above
- Element 1: 1 × 23 = 23.
- Element 2: 1 × 16 = 16; element 3: 1 × 1 = 1.
- Mr = 40 (for NaOH, 23 + 16 + 1 = 40).
Assumptions and limits
- Natural isotope abundances.
- Formula is correct and fully expanded.
Common mistakes
- Forgetting to multiply atoms inside a bracket, as in Ca(OH)₂.
- Using atomic number instead of mass number.
Statistics and uncertainty
Atomic masses are weighted averages over isotopes, so Mr values are exact only for the natural isotope mix.
Where it is used
- Preparing solutions and reagent amounts.
- Checking compound purity and composition.
Maths you need
- Formulae: counting atoms
- Periodic table: relative atomic masses