Practice Test 4 - AP Chemistry

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. A student writes the combustion of methane as $\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}$. Which statement best describes what must be done to satisfy conservation of mass in this reaction equation?
2. A student writes $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ for the synthesis of ammonia. Which statement correctly distinguishes coefficients from subscripts in this equation?
3. Two separate beakers contain equal volumes of water. Beaker 1 has $0.10\,\text{mol}$ of $\text{MgCl}_2(s)$ dissolved. Beaker 2 has $0.10\,\text{mol}$ of $\text{NaCl}(s)$ dissolved. Assuming complete dissociation, which statement best compares the particulate-level composition of the two solutions?
4. At 25°C, the autoionization of water, $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$, is a reversible reaction that establishes equilibrium. Which statement correctly describes pure water at equilibrium?
5. A beaker of a pure liquid is maintained at its boiling point. If heat is continuously added to the beaker at a constant rate, which of the following is expected to be observed?
6. Consider the elements P (atomic number 15), Cl (atomic number 17), and Ar (atomic number 18), which all lie in Period 3. Which element has the greatest electronegativity?
7. A proposed mechanism for the reaction $\text{N}_2\text{O}_5(g)\rightarrow 2\text{NO}_2(g)+\tfrac{1}{2}\text{O}_2(g)$ is: Step 1: $\text{N}_2\text{O}_5(g)\rightarrow \text{NO}_2(g)+\text{NO}_3(g)$ Step 2: $2\text{NO}_3(g)\rightarrow 2\text{NO}_2(g)+\text{O}_2(g)$ Which statement best describes the overall consistency of the mechanism with the net reaction?
8. A student mixes aqueous hydrofluoric acid with water: $\mathrm{HF(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + F^-(aq)}$. Which pair represents a conjugate acid–base pair in this reaction?
9. A two-step reaction has these potential energies: reactants 50 kJ/mol, first transition state (TS1) 150 kJ/mol, intermediate 80 kJ/mol, second transition state (TS2) 120 kJ/mol, and products 20 kJ/mol. Which statement correctly compares $E_a$ for Step 1 and Step 2?
10. Use Hess's law to determine $\Delta H$ for: $\mathrm{2C(s) + 3H_2(g) \rightarrow C_2H_6(g)}$. The following reactions are given: 1) $\mathrm{2C(s) + 2H_2(g) \rightarrow C_2H_4(g)}$ $\Delta H = +52\ \mathrm{kJ}$ 2) $\mathrm{C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)}$ $\Delta H = -149\ \mathrm{kJ}$
11. A reactant $A$ is consumed in the reaction $A \rightarrow$ products. Five concentration–time curves (Curves 1–5) are shown for $[A]$ under different conditions. Each curve levels off when the reaction is complete. Which curve shows a reaction that maintains a relatively high rate for longer (i.e., stays steep for a longer time before gradually leveling off)?
12. A calorimeter has a calorimeter constant of $C = 120\ \text{J}\,^{\circ}\text{C}^{-1}$. During an experiment, the calorimeter temperature increases by $5.0^{\circ}\text{C}$. What is the heat absorbed by the calorimeter, $q$, in joules?
13. A mixture contains small glass beads and a solution of copper(II) sulfate in water. The student wants to separate the beads from the solution while keeping the solution unchanged. Which method is most appropriate?
14. A sample of solid KBr does not conduct electricity, but molten KBr does. Which explanation best accounts for this difference based on ionic solid structure?
15. At 470 nm, a solution in a 1.00 cm cuvette has absorbance $A=0.28$. The concentration is $7.0\times10^{-5}\ \text{M}$. What is the molar absorptivity $\varepsilon$?
16. A student compares two electromagnetic waves. Wave X is in the microwave region, and Wave Y is in the infrared region. Which comparison is correct for photon energy?
17. An ionic solid is formed from ${Mg}^{2+}$ and ${N}^{3-}$ ions arranged in a repeating 3D lattice. Which empirical formula correctly represents a charge-balanced ionic compound made from these ions?
18. A student is given the complete ionic equation: $\mathrm{2\,Na^+(aq) + CO_3^{2-}(aq) + 2\,H^+(aq) + 2\,Cl^-(aq) \rightarrow 2\,Na^+(aq) + 2\,Cl^-(aq) + H_2O(l) + CO_2(g)}$. Which net ionic equation corresponds to it?
19. A student mixes aqueous solutions of silver nitrate, AgNO3(aq), and sodium chloride, NaCl(aq). A white solid forms immediately and settles to the bottom of the container. Which statement best explains whether this change is physical or chemical?
20. A sample of iodine is heated in a closed flask. Purple vapor appears above the remaining solid, and when the flask is cooled, solid iodine reappears on the flask walls. No other substances are introduced. Which statement best classifies the change?

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