What share of a compound's mass is one element, how much metal an ore can give, and how much solid is dissolved in each cubic decimetre of a solution.
What you will learn
- Calculate the percentage by mass of an element in a compound, hydrates included
- Calculate the mass of an element in a given mass of compound
- Calculate concentration, mass or volume using g/dm³, converting cm³ to dm³
Percentage by mass
Once you have the Mr, the share belonging to one element is a fraction of it.
percentage by mass of an element = (number of atoms × Ar) ÷ Mr × 100
- Nitrogen in ammonium nitrate, NH₄NO₃ (Ar: N = 14, H = 1, O = 16)
- Mr = 14 + 4 + 14 + 48 = 80
- mass of N in the formula = 2 × 14 = 28
- percentage = 28 ÷ 80 × 100 = 35%
The same fraction scales up. If 35% of ammonium nitrate is nitrogen, a 20 kg bag holds 20 × 28 ÷ 80 = 7 kg of nitrogen. For water in a hydrate, the "element" is the water: 5 × 18 = 90 out of 249.5 in CuSO₄.5H₂O, which is 36.1%.
Count every atom of the element. Fe₂O₃ has two iron atoms, so use 2 × 56 = 112, not 56.
The rest of this lesson and its 8 practice questions are locked.
In this lesson
- Percentage by mass
- Concentration in g/dm³
- More solute or less water
- Try it: Check a percentage with the calculator
- Try it: Your first percentage
8 practice questions, marked as you go; about 20 minutes in all.
In this unit: Quantitative chemistry and the mole
Conservation of mass, relative formula mass, percentages and concentrations, the mole, reacting masses, empirical formulae, yield, atom economy, titrations and gas volumes.