Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A pure substance contains one type of particle: atoms, molecules or formula units. A compound with two elements is still pure if every particle is the same.
- A mixture contains two or more types of particle. The proportions can vary, so a mixture has no formula.
- A pure compound always has the same mass percent of each element (law of definite proportions).
- A mixture’s mass percent of an element depends on how much of each component it holds. For two components it lies between the two pure values.
- Elemental analysis measures the mass of an element in a sample. Use it to find relative numbers of atoms (via moles) and to judge purity.
Key relationships
| Relationship | Use it to |
|---|---|
| mass fraction of X = (atoms of X × atomic mass of X) ÷ M | find the fixed composition of a pure compound |
| mass of compound = mass of X ÷ mass fraction of X | find how much compound supplied the element |
| purity (%) = mass of compound ÷ mass of sample × 100 | find the percent of the sample that is the compound |
| a + b = moles of X; M₁a + M₂b = total mass | solve a two-component mixture when both parts contain X |
Assumptions
- In the one-step purity method, the impurity contains none of the analysed element.
- The analysis recovers all of the element; losses make the purity look too low.
- Molar masses assume the natural mix of isotopes.
Mistakes to avoid
- Element percent ≠ compound percent. 40.8% Cl in a sample does not mean 40.8% KCl.
- Simple averages. A mixture’s mass percent is a weighted average, not the midpoint.
- “Two elements = mixture.” NaCl is pure; salt plus sand is a mixture.
- Ignoring a second source of the element. If both components contain it, you need two equations.
- Purity over 100%. This signals the impurity contains the element, or a data error.
- Rounding mass fractions early. Keep them unrounded until the last step.
Quick self-check
- What is the mass percent of oxygen in pure MgO? (39.69%)
- A 25.0 g sample of impure MgO contains 12.0 g of Mg. The impurity has no Mg. What is the purity? (79.6% MgO)
- Could a mixture of NaCl and KCl be 65% chlorine by mass? (No: the limits are 47.55% and 60.66%)
- Three samples from one bottle give 31%, 36% and 28% Ca. Pure or mixture? (Mixture: the values vary)
Next: practice questions.