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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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Each FRQ type tests specific skills taught in particular units. Here's why certain units appear for each question type:

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Practice FRQ 1 of 351/35

2. Answer the following questions about strontium and its compounds.

Strontium (Sr) is an alkaline earth metal used in fireworks and magnets. A mass spectrum for a sample of naturally occurring strontium is shown in Figure 1.

Figure 1. Mass spectrum of naturally occurring strontium (Sr): percent abundance vs. mass number

Figure 1
A.
i.

Using the data in Figure 1, identify the isotope of strontium that has the greatest number of neutrons.

ii.

The average atomic mass of strontium listed on the periodic table is 87.62 amu. Explain why this value is not an integer, referencing the data in Figure 1.

A student performs an experiment to determine the empirical formula of a strontium oxide. The student heats a sample of strontium metal in a crucible with excess oxygen. The data collected is shown in Figure 2.

Figure 2. Mass data collected during formation of strontium oxide

Table 2
B.
i.

Using the data in Figure 2, calculate the number of moles of oxygen (O) that reacted with the strontium.

ii.

Determine the empirical formula of the strontium oxide. Justify your answer with a calculation.

The photoelectron spectra (PES) for the 3s sublevel of calcium (Ca) and strontium (Sr) are represented in Figure 3.

Figure 3. Photoelectron spectroscopy (PES) peaks showing binding energy positions used to compare Ca and Sr core/inner-shell electrons

Figure 3
C.
i.

Write the complete ground-state electron configuration for a strontium atom.

ii.

Based on atomic structure, identify which peak in Figure 3 (Peak A or Peak B) represents the 3s electrons of strontium. Justify your answer using Coulomb's law.

D.

Periodic trends can be used to predict properties of strontium compared to other elements.

i.

The first ionization energy of strontium is 549 kJ/mol, while the first ionization energy of calcium is 590 kJ/mol. Explain this difference in terms of atomic structure.

ii.

Predict whether the ionic radius of the Sr²⁺ ion is larger than, smaller than, or equal to the atomic radius of the neutral Sr atom. Justify your answer.

A geologist analyzes a 4.00 g sample of a mixture containing strontium chloride, SrCl₂, and inert impurities. Elemental analysis determines that the mixture contains 0.876 g of strontium (Sr).

E.
i.

Calculate the number of moles of Sr present in the 4.00 g mixture.

ii.

Calculate the mass percent of SrCl₂ in the mixture.

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Free Response Question Practice

This practice environment simulates the AP AP Chemistry Free Response Questions section. Here are some guidelines:

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