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🧪AP Chemistry
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🧪AP Chemistry

FRQs 1–3 – Long Answer
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Unit 1: Atomic Structure and Properties
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This mapping reflects College Board's exam structure - each FRQ type tests specific skills that are taught in particular units.

Practice FRQ 1 of 351/35

3. Answer the following questions about selenium, Se, and its compounds.

Selenium is a nonmetal in Group 16 of the periodic table. It has several naturally occurring isotopes.

Figure 1. Mass spectrum of selenium (Se): relative abundance vs. mass-to-charge ratio (m/z)

Figure 1
A.

The mass spectrum of a sample of pure selenium is shown in Figure 1.

i.

Based on Figure 1, identify the mass number of the most abundant isotope of selenium in the sample.

ii.

The sample contains three major isotopes with the following percent abundances: Se-78 (24.0%), Se-80 (50.0%), and Se-82 (26.0%). Calculate the average atomic mass of selenium in this sample.

A chemist analyzes a 1.50 g sample of pure selenium dioxide, SeO₂.

B.

Calculate the total number of atoms in the 1.50 g sample of SeO₂.

Figure 2. Photoelectron spectrum of selenium (partial): electron binding energy vs. relative number of electrons

Figure 2
C.

The photoelectron spectrum of the valence and near-valence shells of selenium is shown in Figure 2.

i.

Identify the peak (A, B, or C) that corresponds to the 4s electrons. Justify your answer based on relative peak intensity.

ii.

Explain why Peak A is located at a higher binding energy than Peak B.

The atomic radii of sulfur and selenium are listed in Table 1.

Table 1. Atomic Radii

Element

Atomic Radius (pm)

Sulfur (S)

100

Selenium (Se)

115

D.

Using the data in Table 1 and principles of atomic structure, explain why the atomic radius of selenium is larger than the atomic radius of sulfur.

Se(g) → Se⁺(g) + e⁻

IE₁ = 941 kJ/mol
E.

The first ionization energy of arsenic (As) is 947 kJ/mol, while the first ionization energy of selenium (Se) is 941 kJ/mol.

i.

Write the complete ground-state electron configuration for the arsenic atom, As.

ii.

Explain why the first ionization energy of selenium is less than that of arsenic, despite selenium having a higher nuclear charge.

Selenium reacts with fluorine gas to form a compound. In an experiment, 1.58 g of Se(s) reacts completely with excess F₂(g) to produce 3.10 g of the compound.

F.
i.

Calculate the number of moles of selenium and the number of moles of fluorine in the compound formed.

ii.

Determine the empirical formula of the compound.

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