2025 AP Chemistry Free-Response Questions

1. Answer the following questions about magnesium. A. An incomplete mass spectrum for magnesium is shown in the diagram. [Diagram: mass spectrum with y-axis labeled Relative Abundance (0–100) and x-axis labeled Mass (amu) (24, 25, 26), showing a peak at mass 24] The percent abundance of magnesium-24 is 79%. The percent abundances of the other two natural isotopes of magnesium, magnesium-25 and magnesium-26, are approximately equal. i. Complete the mass spectrum in part A by drawing thick lines in the appropriate locations to represent the percent abundance of magnesium-25 and magnesium-26. ii. Describe the difference in atomic structure that accounts for the difference in mass between magnesium-25 and magnesium-26. A student prepares a 1.85×10⁻³ M solution of Mg(NO₃)₂(aq) in beaker 1 and a 2.80×10⁻⁴ M solution of NaOH(aq) in beaker 2, as shown. [Diagram: particle diagram of Mg²⁺ surrounded by water molecules, and two beakers labeled Beaker 1: 1.85×10⁻³ M Mg(NO₃)₂(aq), 35.00 mL; Beaker 2: 2.80×10⁻⁴ M NaOH(aq), 50.00 mL] B. The particle diagram shown represents a magnesium ion, Mg²⁺, in beaker 1. A sodium ion, Na⁺, in beaker 2 has a weaker attraction to water than the Mg²⁺ 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 C. Calculate the pH of the solution in beaker 2. D. A student combines 35.00 mL of 1.85×10⁻³ M Mg(NO₃)₂(aq) with 50.00 mL of 2.80×10⁻⁴ M NaOH(aq), as shown in the diagram. Calculate [Mg²⁺] after the two solutions are combined but before any reaction takes place. (Assume that volumes are additive.) [Diagram: two beakers pouring into a third beaker labeled Beaker 3] E. The dissolution of magnesium hydroxide is represented by the following equation. Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2 OH⁻(aq) Ksp = 5.61×10⁻¹² i. Write the expression for the solubility product constant, Ksp. ii. After the two solutions are combined in beaker 3 as described in part D, but before any reaction takes place, [OH⁻] = 1.65×10⁻⁴ M. Using your answer to part D, calculate the value of the reaction quotient, Q. iii. Using the reaction quotient, Q, predict whether a precipitate should form as the mixture in beaker 3 approaches equilibrium. Justify your answer. F. In a separate experiment, the student adds HNO₃(aq) to decrease the pH of a saturated solution containing undissolved Mg(OH)₂(s). Does the amount of undissolved Mg(OH)₂(s) increase, decrease, or remain the same as the HNO₃(aq) is added? Justify your answer.