Elements & Mixtures (College Board AP® Chemistry): Exam Questions

A laboratory receives a powdered sample containing iron (Fe) and silicon dioxide (SiO₂). The sample is believed to be a mixture of these two substances. To analyze the sample, the following procedures are performed:

• A magnet is used to separate iron from the mixture.

• The remaining material is heated strongly with hydrofluoric acid (HF), which reacts with silicon dioxide according to the equation:

SiO₂ (s) + 4HF (aq) → SiF₄ (g) + 2H₂O (l)

• The total mass of the original sample is 15.00 g. 6.25 g remains after separation with the magnet.

Explain why the original sample is classified as a mixture and not a pure substance.

Calculate the percent by mass of iron in the original sample and explain what this value indicates about the composition of the mixture.

Describe how physical properties are used to separate the components of the mixture.

Explain how the expected observations of the reaction of the remaining material with hydrofluoric acid provide evidence that the other substance could be silicon dioxide.

A laboratory receives a 10.00 g solid sample suspected to be either:

• Pure copper(II) sulfate pentahydrate (CuSO₄·5H₂O), or

• A mixture of copper(II) sulfate pentahydrate (CuSO₄·5H₂O) and an inert, non-volatile impurity.

To analyze the sample, the following procedure is performed:

1. The sample is heated strongly in a crucible to remove all water of hydration. Heating continues until the mass remains constant, with a final mass of 7.85 g.

2. After cooling, the residue is stirred into distilled water. The entire solid dissolves, producing a blue solution.

Explain why the sample is heated until a constant mass is achieved before analysis.

Calculate the percent by mass of water lost during heating.

The theoretical percent by mass of water in pure CuSO₄·5H₂O is 36.1%. Use this value and your calculation from part (b) to justify whether the sample is a mixture or pure copper(II) sulfate pentahydrate.

Explain how this experiment demonstrates that mixtures can have variable composition, based on the mass data.

A municipal water treatment facility is analyzing a 500.0 g sample of river water suspected of containing both suspended solids (such as sand and organic matter) and dissolved ionic compounds.

To investigate, the following procedure is carried out:

1. The sample is filtered, leaving 7.3 g of solid residue on the filter paper.

2. The filtered water is evaporated to dryness, leaving 3.8 g of solid residue.

3. A portion of the evaporated residue is tested with silver nitrate (AgNO₃) solution. A white precipitate forms.

Explain why filtration is an appropriate first step in separating the components of this river water sample.

Calculate the percent by mass of suspended solids in the original river water sample.

What does the result of the test with silver nitrate suggest about the composition of the dissolved solids? Justify your answer.

Explain why the procedures used in this experiment do not allow full identification of all substances present in the river water sample.