AP Chemistry FRQ Practice

🧪AP Chemistry

Practice FRQ 1 of 111/11

1. Answer the following questions about strontium.

A mass spectrum for a sample of strontium is shown in Figure 1. The relative abundances of the four naturally occurring isotopes are: strontium-84 (0.5%), strontium-86 (9.9%), strontium-87 (7.0%), and strontium-88 (82.6%).

Figure 1. Mass spectrum of strontium (Sr): isotope peaks at m/z 84, 86, 87, and 88 with exact relative abundances

A clean, black-and-white mass spectrum displayed as a bar (stick) graph.

Overall layout and axes:
- The plotting area is a wide rectangle.
- Horizontal axis label centered below the axis: "Mass-to-charge ratio (m/z)".
- Horizontal axis runs from 80 to 90, with tick marks and visible numeric tick labels at every integer: 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90.
- Vertical axis label rotated and centered along the left side: "Relative Abundance (%)".
- Vertical axis runs from 0 to 100, with tick marks and visible numeric tick labels every 10%: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100.
- Both axes have arrowheads at the positive ends.
- No gridlines.

Spectrum peaks (must be perfectly vertical bars):
- Exactly four thin, solid black vertical bars (stick peaks). Each bar starts exactly on the x-axis baseline at 0% and extends upward to its stated height.
- Peak 1: a bar centered exactly above the tick labeled 84. The top of this bar aligns exactly with 0.5% (one-half percent). Because the y-axis tick spacing is 10%, the 0.5% height should appear barely above the baseline, clearly nonzero.
- Peak 2: a bar centered exactly above the tick labeled 86. The top of this bar aligns exactly with 9.9% (just below the 10% tick).
- Peak 3: a bar centered exactly above the tick labeled 87. The top of this bar aligns exactly with 7.0% (between 0% and 10%, clearly below the 9.9% bar).
- Peak 4: a bar centered exactly above the tick labeled 88. The top of this bar aligns exactly with 82.6% (between 80% and 90%, clearly closer to 80% than to 90%). This is the tallest bar by a large margin.

Relative-height constraints (enforce visual ranking unambiguously):
- The bar at m/z 88 is dramatically taller than the other three.
- Among the three small peaks between 0% and 10%: the 86 bar is tallest, the 87 bar is the next tallest, and the 84 bar is the shortest.

Optional on-graph numeric annotations (if present, must match exactly):
- Small text directly above each bar top showing the abundance values exactly: "0.5%" above m/z 84, "9.9%" above m/z 86, "7.0%" above m/z 87, and "82.6%" above m/z 88.

No other peaks, curves, shading, or legend.

i. Using the data provided in the background and Figure 1, calculate the average atomic mass of strontium. Show your work.

ii. Explain the difference in atomic structure that accounts for the difference in mass between strontium-88 and strontium-86.

i. The atomic radius of Sr is larger than the atomic radius of Mg.

ii. The first ionization energy of Sr is less than the first ionization energy of Mg.

A student obtains a 2.50 g sample of pure strontium chloride, SrCl₂(s).

C. Calculate the number of moles of SrCl₂ in the 2.50 g sample described above. (The molar mass of SrCl₂ is 158.52 g/mol).

D. The student dissolves the entire 2.50 g sample of SrCl₂ in enough water to make 250.0 mL of solution. Calculate the molar concentration of Sr²⁺(aq) in the solution.

The photoelectron spectrum representing the 3d, 4s, and 4p sublevels of strontium is shown in Figure 2.

Figure 2. Photoelectron spectrum (PES) of strontium (Sr): three peaks labeled A, B, C with binding energy increasing to the left and relative intensities 10:2:6

A black-and-white photoelectron spectrum (PES) shown as a line spectrum with three distinct peaks labeled A, B, and C.

Overall layout and axes (with reversed x-direction):
- The plotting area is a wide rectangle.
- Horizontal axis label centered below the axis: "Binding Energy (MJ/mol)".
- The x-axis direction is explicitly reversed: binding energy increases to the LEFT.
- Place a small note near the x-axis label that reads: "(increases )" using a left-pointing arrow.
- The horizontal axis shows tick marks with visible numeric labels at equal spacing (exactly as text): 6, 5, 4, 3, 2, 1.
- The tick labeled 6 is at the far LEFT end of the axis.
- The tick labeled 1 is at the far RIGHT end of the axis.
- Vertical axis label rotated and centered along the left side: "Relative Intensity (arbitrary units)".
- The y-axis has tick marks and visible numeric labels: 0, 2, 4, 6, 8, 10.
- No gridlines.

Baseline and peak style:
- A thin horizontal baseline coincides exactly with y = 0 across the entire width.
- Each peak is a smooth, symmetric bell-shaped curve (Gaussian-like), drawn with a solid black line, and each peak returns to the baseline (y = 0) on both sides before the next peak begins.
- Peaks do not overlap; there is clear baseline separation between them.

Peak positions and ordering (must match the reversed x-axis):
- Because binding energy increases to the left, the highest-binding-energy peak is the LEFTMOST peak.
- Place three peaks from left to right in this exact order: Peak A (leftmost), Peak B (middle), Peak C (rightmost).

Exact peak heights (relative intensity values must match the y-axis numbers):
- Peak A: maximum height reaches exactly y = 10. The label "A" is placed directly above the apex.
- Peak B: maximum height reaches exactly y = 2. The label "B" is placed directly above the apex.
- Peak C: maximum height reaches exactly y = 6. The label "C" is placed directly above the apex.

Exact horizontal placement using labeled ticks (relative positioning, no coordinates):
- Peak A apex is positioned directly above the tick labeled 5 on the binding-energy axis (left half of the graph).
- Peak B apex is positioned directly above the tick labeled 3 (near the center of the graph).
- Peak C apex is positioned directly above the tick labeled 2 (right-of-center, closer to the right end than the center).

Visual proportionality constraints:
- Peak A is exactly 5 times as tall as Peak B (10 vs 2).
- Peak C is exactly 3 times as tall as Peak B (6 vs 2).
- Peak A is taller than Peak C by exactly 4 intensity units (10 vs 6), which should be visually obvious using the y-axis ticks.

No additional peaks outside A, B, and C.

Important: Do NOT label sublevels (3d, 4s, 4p) on the figure; only label peaks A, B, and C (to match the prompt that asks students to identify which peak corresponds to 4p).

i. Identify the peak in Figure 2 that corresponds to the electrons in the 4p sublevel. Justify your answer based on relative peak intensity.

ii. Explain why the peak corresponding to the 3d sublevel is located to the left of the peak corresponding to the 4s sublevel.

iii. Write the complete ground-state electron configuration for the Sr²⁺ ion.

In a separate experiment, a student analyzes a sample of hydrated strontium chloride, SrCl₂·xH₂O, to determine the value of x.

SrCl₂·xH₂O(s) → SrCl₂(s) + xH₂O(g)

F. The student heats a sample of SrCl₂·xH₂O in a crucible to drive off the water. However, the student stops heating before the crucible has reached a constant mass, leaving some moisture in the sample. Will the calculated value of x be greater than, less than, or equal to the actual value? Justify your answer.