AP Chemistry FRQ Practice

🧪AP Chemistry

Practice FRQ 1 of 351/35

1. Answer the following questions about lithium.

Lithium is a soft, silvery-white alkali metal. It has two naturally occurring isotopes, lithium-6 and lithium-7.

The mass spectrum of a sample of lithium is shown in Figure 1. The percent abundances are 7.6% for lithium-6 (mass 6.02 amu) and 92.4% for lithium-7 (mass 7.02 amu).

i. Calculate the average atomic mass of lithium based on the data provided.

ii. Describe the difference in atomic structure that accounts for the difference in mass between lithium-6 and lithium-7.

Lithium reacts vigorously with water to form lithium hydroxide and hydrogen gas. A student compares the interaction of lithium ions, Li⁺, and magnesium ions, Mg²⁺, with water molecules.

Figure 2 represents a hydrated lithium ion and a hydrated magnesium ion. The Mg²⁺ ion has a stronger attraction to water molecules than the Li⁺ ion does. Explain this phenomenon using Coulomb's law and each of the following.

i. The relative charge of the ions

ii. The relative radii of the ions

2Li(s) + 2H₂O(l) → 2Li⁺(aq) + 2OH⁻(aq) + H₂(g)

C. Using the balanced equation above, calculate the mass of Li(s) (molar mass 6.94 g/mol) required to produce 0.500 mol of H₂(g). Assume excess water.

A student analyzes a 1.50 g sample of a pure compound containing lithium, carbon, and oxygen. The sample is found to contain 0.28 g of lithium, 0.24 g of carbon, and 0.98 g of oxygen.

D. Determine the empirical formula of the compound described. Justify your answer with calculations.

The photoelectron spectrum of lithium is shown in Figure 3.

Figure 1. Mass spectrum of lithium showing the two naturally occurring isotopes at m/z 6 and m/z 7 with their exact percent abundances.

A clean, black-and-white stick (bar) mass spectrum on a plain white background.

Overall layout:
- The plot area is a wide rectangle. No title inside the graph.
- Two axes meet at the lower-left corner (the origin).
- Both axes end with arrowheads at the positive directions.

X-axis (horizontal):
- Axis label centered below the axis: “Mass-to-charge ratio (m/z)”.
- The x-axis numerical range shown spans from 5 to 8.
- Visible tick marks and tick labels appear at 5, 6, 7, and 8 (all four numbers must be printed).
- Tick spacing is uniform so that 6 is exactly one tick interval to the right of 5, 7 is exactly one tick interval to the right of 6, and 8 is exactly one tick interval to the right of 7.

Y-axis (vertical):
- Axis label rotated vertically and centered along the y-axis: “Relative abundance (%)”.
- The y-axis numerical range shown spans from 0 to 100.
- Visible tick marks and tick labels appear at 0, 20, 40, 60, 80, and 100 (all six numbers must be printed).
- Tick spacing is uniform.

Spectrum bars (discrete vertical bars):
- Exactly two vertical rectangular bars are present—no other peaks or noise.
- Each bar is centered exactly above its corresponding integer m/z tick.
- Bar at m/z = 6:
- Positioned directly above the tick labeled “6”.
- Bar height reaches exactly to the y-value labeled 7.6% (render this by placing the top of the bar slightly above the baseline, clearly below the 20% tick, and annotate the numeric value “7.6” directly above the bar top).
- A small text label “7.6%” is printed immediately above the top of this bar.
- Bar at m/z = 7:
- Positioned directly above the tick labeled “7”.
- Bar height reaches exactly to 92.4% (render this by placing the top of the bar between the 80% and 100% ticks, clearly closer to 100% than to 80%, and annotate the numeric value “92.4%” directly above the bar top).
- A small text label “92.4%” is printed immediately above the top of this bar.

Styling constraints to prevent ambiguity:
- Both bars have identical widths (each bar width visually matches the width of the tick label numbers), solid black fill, and no outlines beyond the filled rectangle edge.
- The baseline at y = 0 is a solid axis line; bars start exactly at y = 0 with no gap.
- No gridlines.
- No legend.
- The only peak labels are the numeric percent labels “7.6%” and “92.4%”.

Figure 2. Hydration shells of Li⁺ and Mg²⁺ showing water orientation and coordination number (4 around Li⁺, 6 around Mg²⁺).

A side-by-side particle diagram of two separate hydrated cations on a white background, drawn as two clearly separated clusters with equal vertical alignment.

Overall layout:
- The left cluster depicts Li⁺(aq); the right cluster depicts Mg²⁺(aq).
- The two clusters are separated by a clear blank gap approximately equal to the diameter of one full hydration cluster, ensuring they do not overlap visually.
- Each cluster has a central ion sphere and surrounding water molecules arranged symmetrically.

Central ions (must be visibly labeled):
- Left ion: a single solid sphere labeled “Li⁺” in black text placed just above and slightly to the left of the central sphere.
- Right ion: a single solid sphere labeled “Mg²⁺” in black text placed just above and slightly to the right of the central sphere.
- Relative ion sizes: the Mg²⁺ central sphere is drawn with a smaller diameter than the Li⁺ central sphere (Mg²⁺ is clearly smaller; ensure the diameter difference is visually obvious, not subtle).

Water molecule graphic convention (used identically in both clusters):
- Each water molecule is a three-sphere bent shape:
- One larger sphere for oxygen labeled with the letter “O” on the sphere.
- Two smaller spheres for hydrogens labeled “H” on each small sphere.
- Within each water molecule, the two H spheres are attached to the O sphere in a V shape (bent), with the H–O–H opening facing away from the central ion.

Orientation rule (critical):
- In every water molecule, the oxygen (O) sphere points toward the central ion (O is the closest atom to the ion), and the two hydrogen spheres point away from the ion (H farther from the ion than O).

Left cluster: Li⁺ hydration (coordination number 4):
- Exactly four water molecules surround Li⁺.
- Arrangement: place the four oxygen atoms at the top, bottom, left, and right directions around Li⁺, forming a symmetric “cross” (like cardinal directions).
- Each oxygen sphere is the closest part of its water molecule to Li⁺, and each oxygen is the same distance from the Li⁺ center (all four O–Li⁺ separations equal within the drawing).
- The two hydrogens for each water are drawn on the side of the oxygen facing away from Li⁺, so no hydrogen appears between O and Li⁺.

Right cluster: Mg²⁺ hydration (coordination number 6):
- Exactly six water molecules surround Mg²⁺.
- Arrangement: place six oxygen atoms evenly spaced around Mg²⁺ like points on a hexagon (one at the top, one at bottom, and four at the diagonal positions), creating a ring of six waters.
- All six oxygen spheres are the same distance from the Mg²⁺ center (all six O–Mg²⁺ separations equal within the drawing).
- The two hydrogens for each water are drawn on the side of the oxygen facing away from Mg²⁺, so no hydrogen appears between O and Mg²⁺.

Ion–water distance comparison (must be unambiguous):
- The oxygen spheres in the Mg²⁺ cluster are drawn closer to the Mg²⁺ ion than the oxygen spheres are to the Li⁺ ion in the Li⁺ cluster.
- Implement this visually by making the Mg²⁺ hydration ring tighter: the gap between Mg²⁺ and each O is visibly smaller than the gap between Li⁺ and each O.

No extra elements:
- No anions, no free water molecules outside the two hydration shells, no arrows, and no background solvent dots.
- Only the labels “Li⁺”, “Mg²⁺”, and the atom letters “O” and “H” appear as text in the figure.

Figure 3. Photoelectron spectrum (PES) of lithium with two peaks at binding energies 6.26 MJ/mol and 0.52 MJ/mol and relative electron counts 2 and 1.

A two-peak photoelectron spectrum on a white background, drawn as a line plot with two distinct, non-overlapping peaks.

Overall layout:
- Rectangular plot area with axes meeting at the lower-left corner.
- Arrowheads on the positive ends of both axes.
- No title inside the graph.

X-axis (horizontal):
- Axis label centered below: “Binding Energy (MJ/mol)”.
- The axis is reversed (values increase from right to left): the printed tick labels, from left to right, read “10, 8, 6, 4, 2, 0”.
- Tick marks are evenly spaced; each adjacent pair differs by 2 MJ/mol.
- Ensure the leftmost printed tick is “10” and the rightmost printed tick is “0”.

Y-axis (vertical):
- Axis label rotated vertically and centered along the y-axis: “Relative Number of Electrons”.
- Y-axis shows a baseline at 0.
- Include visible tick labels at 0, 1, and 2 (uniform vertical spacing), since peak heights must match 1 and 2 exactly.

Peaks (shape and placement):
- Use sharp, symmetric triangular peaks (not rounded Gaussians) to make the peak maxima unambiguous.
- Peak A:
- Place a bold letter label “A” directly above the apex of the left-side peak.
- The apex is exactly aligned above the x-axis value 6.26 MJ/mol (print the value “6.26” directly below the apex on the x-axis as a small annotation, separate from the main tick labels).
- The apex height reaches exactly the y-tick labeled 2.
- The triangle’s left and right sides slope linearly down from the apex to the baseline (y = 0), forming a narrow peak that does not touch or overlap the other peak.
- Peak B:
- Place a bold letter label “B” directly above the apex of the right-side peak.
- The apex is exactly aligned above the x-axis value 0.52 MJ/mol (print the value “0.52” directly below the apex on the x-axis as a small annotation).
- The apex height reaches exactly the y-tick labeled 1.
- The triangle’s left and right sides slope linearly down to the baseline (y = 0).

Relative horizontal placement (must reflect reversed axis):
- Because binding energy increases to the left, Peak A at 6.26 MJ/mol must appear left of Peak B at 0.52 MJ/mol.
- Peak B must be positioned close to the right end of the axis (near the 0 tick) but not on top of the 0 tick label.
- Peak A must be positioned between the 6 and 8 ticks, closer to the 6 tick than to the 8 tick.

Styling constraints:
- Peaks drawn with a single solid black line of medium thickness.
- No filled area under peaks.
- No gridlines.
- No legend.
- The only text inside the plot besides axis labels and tick labels is: “A”, “B”, “6.26”, and “0.52”.

i. Write the complete ground-state electron configuration for the lithium atom.

ii. Identify the peak in Figure 3 that corresponds to the 1s electrons. Justify your answer.

iii. The binding energy of the 1s electrons in a beryllium atom (Be) is 11.5 MJ/mol. Explain why the binding energy of the 1s electrons in lithium is lower than that of beryllium.

In a separate experiment, a student analyzes a 2.00 g mixture containing LiCl(s) and an inert solid. The student dissolves the mixture in water and adds excess AgNO₃(aq) to precipitate all the chloride ions as AgCl(s). The precipitate is filtered, dried, and weighed.

F. If the precipitate is not completely dried before weighing, will the calculated mass percent of LiCl in the mixture be greater than, less than, or equal to the actual mass percent? Justify your answer.