AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/1/1-4)
Unit 1 · Topic 1.4
1.4 Composition of Mixtures
A mixture contains two or more substances in proportions that can change from sample to sample. If you measure how much of one element a sample contains, you can work out how much of each substance is present, or how pure the sample is.
Key terms
- mixture
- pure substance
- elemental analysis
- purity
- percent by mass
Pure substances versus mixtures
A pure substance has one kind of atom, molecule or formula unit, and its composition is fixed. A mixture has two or more kinds, and the amounts can vary. Salt water can be a little salty or very salty; pure water is always H₂O.
That difference is useful. Because a pure compound has a known percent of each element (topic 1.3), measuring the actual percent in a sample tells you whether it's pure. If it doesn't match, something else is mixed in.
Elemental analysis
Elemental analysis means measuring how much of a particular element a sample contains. A common way is to make that element react to form a new solid, then weigh the solid. For example, adding excess silver nitrate to a dissolved chloride sample turns all the chloride into solid AgCl, which you can filter, dry and weigh.
From that mass you can work backward: grams of the element → moles of the element → moles of the compound that held it → grams of that compound. Comparing those grams to the total sample mass gives the percent purity.
Reasoning about impurities
Many exam questions don't need a full calculation. Compare the measured percent of an element with the percent in the pure compound, then ask which impurity would push it in that direction.
An impurity that contains none of the element (like sand) always lowers its percent. An impurity that contains a higher percent of the element raises it. An impurity with a lower percent lowers it.
| Compound | Percent Cl by mass |
|---|---|
| LiCl | 83.6% |
| NaCl | 60.7% |
| KCl | 47.6% |
Percent by mass in this topic
Here, percent by mass describes what fraction of a solid sample is a given substance: (mass of the substance ÷ mass of the whole sample) × 100. Unit 3 notes that percent by mass of solutions isn't tested, but percent composition and purity of samples is fair game in this topic.
How lab errors change the result
Exam questions often ask what a mistake does to the answer. Follow the error through the calculation one step at a time.
If a precipitate like AgCl isn't fully dried, the extra water adds mass, so the calculated mass of chlorine, and the percent of chloride compound, come out too high. If some precipitate is lost while filtering, they come out too low. If an impurity also forms a precipitate with silver ions, it adds mass too, and the result is too high.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Percent purity from elemental analysis
A 2.000 g sample is a mixture of NaCl and sand. Analysis shows it contains 0.7260 g of chlorine. What is the percent NaCl by mass in the sample?
Show the solutionHide the solution
- Step 1: All the chlorine came from NaCl, because sand (SiO₂) contains none.
- Step 2: Moles of Cl = 0.7260 g ÷ 35.45 g/mol = 0.02048 mol.
- Step 3: Each NaCl has one Cl, so moles NaCl = 0.02048 mol.
- Step 4: Mass of NaCl = 0.02048 mol × 58.44 g/mol = 1.197 g.
- Step 5: Percent NaCl = 1.197 ÷ 2.000 × 100 = 59.84%.
Answer: 59.84% NaCl
- Example 2Calculator allowed
Which impurity fits the data?
A sample labeled 'pure NaCl' is found to be 63.0% chlorine by mass. Pure NaCl is 60.7% Cl. Could the impurity be KCl, or must it be LiCl? Justify your answer.
Show the solutionHide the solution
- Step 1: The sample has more chlorine by mass than pure NaCl, so the impurity must have a higher percent Cl than 60.7%.
- Step 2: Percent Cl in KCl = 35.45 ÷ 74.55 × 100 = 47.6%. Mixing it in would lower the percent Cl, not raise it.
- Step 3: Percent Cl in LiCl = 35.45 ÷ 42.39 × 100 = 83.6%. Mixing it in raises the percent Cl.
- Step 4: Why: lithium is much lighter than sodium, so LiCl packs more chlorine into each gram.
Answer: The impurity must be LiCl; a KCl impurity would make the percent Cl lower than 60.7%, not higher.
Common mistakes
- Dividing by the mass of the pure compound instead of the whole sample when finding percent purity.
- Assuming every impurity lowers the percent of an element. An impurity richer in that element raises it.
- Forgetting to use the formula ratio. If the compound were CaCl₂, each mole of compound holds 2 moles of Cl, so moles of compound = moles Cl ÷ 2.
On the exam
- Expect short free-response parts that give a lab measurement and ask for percent by mass or purity, followed by a question about how an error (such as a wet precipitate that weighs too much) would change the result. State the direction (too high or too low) and explain why.
- For 'which impurity' questions, calculate the percent of the element in each candidate compound and compare.
Connected topics
Videos
Check yourself
4 questions on 1.4 Composition of Mixtures. Pick an answer to see if you got it, and why.
A 10.0 g mixture contains only NaCl (molar mass 58.44 g/mol) and sand, SiO₂. Elemental analysis shows that the mixture contains 2.43 g of chlorine. What is the percent by mass of NaCl in the mixture?
Pure NaCl is 60.7% chlorine by mass. A sample of NaCl contaminated with a single impurity is found to be 62.5% chlorine by mass. Which of the following could be the impurity?
A 10.00 g mixture contains only NaCl (molar mass 58.44 g/mol) and KCl (molar mass 74.55 g/mol). Elemental analysis shows that the mixture contains 5.50 g of chlorine. What mass of NaCl is in the mixture?
An iron ore is 80.0% Fe₂O₃ (molar mass 159.69 g/mol) by mass. The rest of the ore contains no iron. What is the percent by mass of iron in the ore?
0 of 4 answered