Practice Test 7 - 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 mixes aqueous solutions of calcium chloride and sodium carbonate and observes a white precipitate form. Which balanced molecular equation best represents the reaction that occurs (including physical states)?
2. A student investigates the reaction between aqueous hydrochloric acid and magnesium metal, which produces hydrogen gas and dissolved magnesium ions. Two trials use the same total volume of solution and the same mass of Mg(s), and no catalyst is present. Condition 1: Mg(s) is added to a dilute HCl(aq) solution. Condition 2: Mg(s) is added to a more concentrated HCl(aq) solution. The student is told that the reaction mechanism is unchanged between the two conditions and that the only difference is how often reactant particles collide at the metal surface. Which statement best explains why the reaction occurs faster in Condition 2 than in Condition 1, using collision-model reasoning?​
3. A student prepares a nonreacting alloy by mixing $60.0\,\text{g}$ of Cu and $40.0\,\text{g}$ of Zn. What is the mass percent of Zn in the alloy?
4. A student prepares a buffer by mixing aqueous acetic acid, $\mathrm{HC_2H_3O_2}$, with sodium acetate, $\mathrm{NaC_2H_3O_2}$. A small amount of strong acid, $\mathrm{HCl(aq)}$, is added to the buffer. Which statement best explains how and why the buffer resists a large pH change, including which component reacts with the added acid?
5. A piece of calcium metal reacts with water to produce calcium hydroxide and hydrogen gas: $\text{Ca}(s)+2\text{H}_2\text{O}(l)\rightarrow \text{Ca(OH)}_2(aq)+\text{H}_2(g)$. Which reaction type best classifies this reaction?
6. A galvanic cell is constructed with the overall reaction $\text{H}_2(g) + \text{Cu}^{2+}(aq) \rightarrow 2\text{H}^+(aq) + \text{Cu}(s)$ At standard conditions ($P_{\text{H}_2}=1\,\text{atm}$, $[\text{Cu}^{2+}]=1\,\text{M}$, and $[\text{H}^+]=1\,\text{M}$), the cell is spontaneous. The cell is prepared with $[\text{H}^+]$ much smaller than $1\,\text{M}$ while $P_{\text{H}_2}$ and $[\text{Cu}^{2+}]$ remain at their standard-state values. Using qualitative Nernst reasoning about how $Q$ changes, how does $E_{\text{cell}}$ change and does the reaction remain spontaneous?​
7. A student titrates $25.0\,\text{mL}$ of $0.200\,\text{M}$ ammonia, $\text{NH}_3$ ($K_b = 1.8\times10^{-5}$; $pK_b\approx 4.74$), with $0.100\,\text{M}$ HCl. At the half-equivalence point, which statement best describes the solution?
8. A student investigates the precipitation reaction that occurs when aqueous solutions of $\text{AgNO}_3$ and $\text{NaCl}$ are mixed, forming $\text{AgCl}(s)$. The mechanism is unchanged, and the only difference between trials is how frequently reactant ions encounter each other after mixing. Condition 1: Both solutions are relatively dilute before mixing. Condition 2: Both solutions are more concentrated before mixing. Both mixtures are stirred in the same way and kept at the same temperature. Which statement best explains why the precipitate forms faster in Condition 2 than in Condition 1 using collision-model reasoning?
9. A buffer is prepared by mixing $\mathrm{H_2PO_4^-}$ and $\mathrm{HPO_4^{2-}}$ in water so that both species are present. A small amount of $\mathrm{HCl(aq)}$ is added. Which statement best describes the primary reaction that helps resist the pH change?
10. Solid potassium hydrogen tartrate, $\text{KHC}_4\text{H}_4\text{O}_6(s)$, dissolves to produce $\text{K}^+$ and $\text{HC}_4\text{H}_4\text{O}_6^-$. The anion is acidic and can lose $\text{H}^+$. If the pH is increased by adding NaOH(aq), what happens to the solubility and why?
11. A buffer solution contains HCN(aq) and NaCN(aq). A small amount of HCl(aq) is added. Which statement best explains how the buffer resists pH change and identifies the species that reacts with the added acid?
12. Two buffers are prepared using the same weak acid/conjugate base pair, HNO$_2$/NO$_2^-$. Each buffer is made by mixing solutions to a total volume of 1.00 L. - Buffer S: 0.050 mol HNO$_2$ and 0.150 mol NO$_2^-$ - Buffer T: 0.50 mol HNO$_2$ and 1.50 mol NO$_2^-$ The same small amount of strong acid is added to both buffers. Which statement best compares the buffer capacities of S and T for this addition?
13. A $2.00\,\text{mol}$ sample of an ideal gas is in a $10.0\,\text{L}$ container at a pressure of $4.92\,\text{atm}$. What is the temperature in K? (Use $R = 0.0821\,\text{L}\cdot\text{atm}\cdot\text{mol}^{-1}\cdot\text{K}^{-1}$.)
14. A student investigates the solubility of lead(II) sulfide, $\text{PbS}(s)$, in water and in 0.10 M HNO$_3$(aq). Which prediction best describes the effect of lowering pH on the solubility of $\text{PbS}$ and why?
15. Four carboxylic acids have substituents on the carbon adjacent to the COOH group: I: CH3CH2COOH II: CH3CHClCOOH III: CH3CHFCOOH IV: CH3CHBrCOOH Which ranks the acids from weakest to strongest based on inductive effects?
16. Which of the following statements provides the most accurate and fundamental definition of an oxidation-reduction (redox) reaction?
17. Two covalent bonds are compared: a C–H bond and a C–C bond, both single bonds. Which bond is expected to have the greater bond strength in general?
18. A proposed mechanism for the overall reaction $\text{2SO}_2(g)+\text{O}_2(g)\rightarrow 2\text{SO}_3(g)$ is: Step 1: $\text{SO}_2(g)+\text{O}_2(g)\rightarrow \text{SO}_4(g)$ Step 2: $\text{SO}_4(g)+\text{SO}_2(g)\rightarrow 2\text{SO}_3(g)$ Which species is an intermediate in the mechanism?
19. Consider $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$. Which statement best describes what the subscript 3 in $\mathrm{NH_3}$ indicates?
20. A student studies the reversible reaction $\text{CH}_3\text{COOH}(aq) \rightleftharpoons \text{H}^+(aq) + \text{CH}_3\text{COO}^-(aq)$ in a closed system at constant temperature. After sufficient time, the pH remains constant. Which statement best explains what equilibrium means for this system?

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