Library / Foundations: Chemistry / FCH-02
Foundations: Chemistry
Percentage by mass of an element
The share of a compound’s mass that comes from one element.
- Ref
- FCH-02
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- % [%]
n is the number of atoms of that element in the formula, Ar its atomic mass and Mr the relative formula mass of the whole compound.
It tells you how much of the compound, by weight, is that element.
Calculator
- Mass of element in 100 g60 g
- Mass of the rest in 100 g40 g
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.
- Magnesium makes up 60% of magnesium oxide by mass, so 10 g of MgO contains 6 g of Mg.
- The percentages of all elements in a compound add to 100%.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (%) | Local step | Change in result |
|---|---|---|---|---|---|---|
| n Atoms of the element | 1 to 20 | 60 | 1,200 | 1,140 | +10% | +10 % |
| Ar Atomic mass of the element | 1 to 240 | 2.5 | 600 | 597.5 | +10% | +10 % |
| Mr Relative formula mass | 10 to 500 | 4.8 | 240 | 235.2 | +10% | −9.09 % |
Sweep of Relative formula mass
| Relative formula mass | Mass percentage (%) |
|---|---|
| 10 | 240 |
| 16.31 | 147.2 |
| 26.59 | 90.25 |
| 43.36 | 55.35 |
| 70.71 | 33.94 |
| 115.3 | 20.81 |
| 188 | 12.76 |
| 306.6 | 7.827 |
| 500 | 4.8 |
Sweep one variable
Variables and how each one contributes
Mass percentage
What the equation gives you. Everything else feeds it.
Atoms of the element
Number of atoms of the element in one formula unit. Raises the percentage in proportion.
Atomic mass of the element
Relative atomic mass of the element (Mg = 24). Raises the percentage in proportion.
Relative formula mass
Relative formula mass of the compound (MgO = 40). Lowers the percentage as the compound gets heavier.
Worked example with the values above
- Mass of the element in one formula unit: 1 × 24 = 24.
- Fraction of the whole: 24 / 40 = 0.6.
- Percentage = 60 %.
Assumptions and limits
- Pure compound.
- The element’s atoms are counted per formula unit.
Common mistakes
- Dividing by Ar of the element instead of Mr of the compound.
- Forgetting the factor n.
Statistics and uncertainty
If Ar and Mr are rounded to whole numbers the result is good to about 1%.
Where it is used
- Ore grade and metal content.
- Purity and fertiliser nitrogen content.
Maths you need
- Relative formula mass: sum of Ar
- Percentages: fraction × 100