AP Chemistry 1.4.A: Composition of Mixtures

Explore AP Chemistry 1.4.A questions on distinguishing pure substances from mixtures and using elemental analysis to calculate composition and purity.

[Maximum number: 2]

Answer the following questions relating to Fe and its ions, $\mathrm{Fe}^{2+}$ and $\mathrm{Fe}^{3+}$ .

(a)

Calculate the percent by mass of Fe in the original sample of powdered Fe(s) with the inert impurity.

[ 1 ]

Calculate the percent by mass of Fe in the original sample of powdered Fe(s) with the inert impurity.

$\begin{gathered} 0.09431 \mathrm{~mol} \mathrm{Fe} \times \frac{55.85 \mathrm{~g} \mathrm{Fe}}{1 \mathrm{~mol}}=5.267 \mathrm{~g} \mathrm{Fe} \frac{5.267 \mathrm{~g} \mathrm{Fe}}{6.724 \mathrm{~g} \text { sample }} \times 100=78.33 \% \end{gathered}$

1 point is earned for correct calculation of the
mass percent based on the answer to part (g).

(b)

If the oxidation of the Fe(s) in the original sample was incomplete so that some of the 7.531 g of product was FeO(s) instead of $\mathrm{Fe}_{2} \mathrm{O}_{3}(s)$ , would the calculated mass percent of Fe(s) in the original sample be higher, lower, or the same as the actual mass percent of $\operatorname{Fe}(s)$ ? Justify your answer.

Sulfur atom = Carbon atom = Oxygen atom =

[ 1 ]

The calculated mass percent of Fe would be lower than the actual mass percent of Fe.

A sample that contains any FeO (rather than $\mathrm{Fe}_{2} \mathrm{O}_{3}$ ) will have a higher actual mass percent of Fe than a completely oxidized sample would have. Therefore, when the moles of Fe are calculated (assuming all the mass of the sample is $\mathrm{Fe}_{2} \mathrm{O}_{3}$ ) the calculated number of moles of Fe, and hence the calculated mass percent of Fe, will be lower.

1 point is earned for the correct answer and a valid explanation.

Sulfur atom = Carbon atom = Oxygen atom =

Compound

Molecular Structure

Boiling Point at 1 atm
(K)

$\mathrm{CS}_{2}$

Figure for Answer to Question (b) — AP Chemistry

COS

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1.4.A—Explain the quantitative relationship between the elemental composition by mass and the composition of… question 1

Question (a)

Question (b)