Practice Test 8 - AP Chemistry

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

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1. A saturated solution of $\text{AgCl}(s)$ is at equilibrium with undissolved solid: $\text{AgCl}(s) \rightleftharpoons \text{Ag}^+(aq)+\text{Cl}^-(aq)$. A small amount of $\text{NaCl}(s)$ is added and dissolves. Assuming temperature is constant and solid $\text{AgCl}$ remains present, how does the solubility of $\text{AgCl}$ change?
2. Excess solid barium sulfate, $\text{BaSO}_4(s)$, is stirred in pure water at $25^\circ\text{C}$ until equilibrium is reached. The $K_{sp}$ of $\text{BaSO}_4$ is $1.0\times10^{-10}$. Dissolution: $\text{BaSO}_4(s)\rightleftharpoons \text{Ba}^{2+}(aq)+\text{SO}_4^{2-}(aq)$ Starting from zero ion concentrations, what is the equilibrium concentration of $\text{Ba}^{2+}(aq)$? ICE table (in M):

   	$\text{Ba}^{2+}$	$\text{SO}_4^{2-}$
   I	0	0
   C	+$s$	+$s$
   E	$s$	$s$

3. A mass spectrum of element M shows peaks at $m/z=63$ (relative abundance 69) and $m/z=65$ (relative abundance 31). Assuming $+1$ ions, which value is closest to the average atomic mass of M?
4. A student has a mixture of two liquids that form two layers in a separatory funnel: water (bottom layer) and diethyl ether (top layer). The student wants to separate the two liquids. Which method is most appropriate?
5. A sample of sulfur hexafluoride, $\mathrm{SF_6}$, is held at high pressure in a container at room temperature. A student calculates the compressibility factor $Z=\dfrac{PV}{nRT}$ and finds $Z>1$. Which interpretation best explains this result in terms of real-gas behavior?
6. A rigid container holds CO$_2$(g) at 1.0 atm and 298 K. The gas is cooled at constant volume until some CO$_2$ deposits as solid (dry ice), leaving a mixture of solid CO$_2$ and CO$_2$(g). From initial state (all gas) to final state (some solid present), how does the entropy of the CO$_2$ change?
7. A student connects two half-cells under standard conditions: MnO$_4^-$(aq), Mn$^{2+}$(aq) in acidic solution with an inert electrode, and Sn(s)|Sn$^{2+}$(aq). Standard reduction potentials: MnO$_4^-$ + 8H$^+$ + 5e$^-$ 6 Mn$^{2+}$ + 4H$_2$O, $E^\circ = +1.51\,\text{V}$; Sn$^{2+}$ + 2e$^-$ 6 Sn(s), $E^\circ = -0.14\,\text{V}$. If MnO$_4^-$ is reduced at the cathode, which statement is correct?
8. A student seals a flask containing the reversible reaction $\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$ and lets it sit at constant temperature. After some time, the observed color intensity of the gas mixture remains unchanged. Which statement best describes what it means for this system to be at equilibrium at the particle/rate level?
9. For the equilibrium reaction $2\text{NO}_2(g)\rightleftharpoons 2\text{NO}(g)+\text{O}_2(g)$, the equilibrium constant is $K$. What is $K_{\text{new}}$ for the reaction $\text{NO}(g)+\tfrac{1}{2}\text{O}_2(g)\rightleftharpoons \text{NO}_2(g)$?
10. A metallic solid and an ionic solid are both heated above their melting points. Which statement correctly compares their electrical conductivity in the molten state?
11. Excess solid zinc sulfide, $\text{ZnS}(s)$, is added to pure water at $25^\circ\text{C}$. The equilibrium is $\text{ZnS}(s) \rightleftharpoons \text{Zn}^{2+}(aq) + \text{S}^{2-}(aq)$ with $K_{sp}=1.0\times10^{-24}$. What is the molar solubility of $\text{ZnS}$ (in $\text{mol}\cdot\text{L}^{-1}$)?
12. For the reaction $\text{CaCO}_3(s)\rightarrow \text{CaO}(s)+\text{CO}_2(g)$, $\Delta H=+178\ \text{kJ}$ as written. What does this imply about heat flow?
13. Magnesium reacts with nitrogen gas to form magnesium nitride: $\mathrm{3\,Mg(s) + N_2(g) \rightarrow Mg_3N_2(s)}$ If $0.90\ \text{mol}$ of $\mathrm{Mg}$ reacts completely with excess $\mathrm{N_2}$, how many moles of $\mathrm{Mg_3N_2}$ are formed?
14. Which of the following is the most precise and fundamental definition of chemical equilibrium?
15. A sealed, rigid container is divided into two equal compartments by a valve. The left side contains 1.0 mol of Ne(g) at 298 K, and the right side is a vacuum. The valve is opened and the gas expands to fill the entire container while the temperature remains approximately constant. For the system (the gas), does entropy increase, decrease, or remain approximately constant?
16. A galvanic cell operates with the overall reaction $\text{Sn}(s) + 2\text{H}^+(aq) \rightarrow \text{Sn}^{2+}(aq) + \text{H}_2(g)$ At standard conditions ($[\text{H}^+]=1\,\text{M}$ and $P_{\text{H}_2}=1\,\text{atm}$), the cell is spontaneous. The cell is prepared with $P_{\text{H}_2}$ much larger than $1\,\text{atm}$ while $[\text{H}^+]$ and $[\text{Sn}^{2+}]$ are each $1\,\text{M}$. Using qualitative Nernst reasoning about how $Q$ changes, how does $E_{\text{cell}}$ change and does the reaction remain spontaneous?​
17. In the reaction $\mathrm{H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)}$, which statement correctly identifies the Brønsted–Lowry roles of water?
18. A closed system contains the reversible gas-phase reaction $2\text{NO}(g)+\text{Cl}_2(g) \rightleftharpoons 2\text{NOCl}(g)$. A student measures concentrations early in the run, before any concentrations become constant. Based only on the table, what is the direction of the net reaction progress during the interval shown?
19. A $24\,\text{g}$ sample of ice melts at $0^\circ\text{C}$. For water, $\Delta H_{\text{fus}}=334\,\text{J/g}$. How much energy is absorbed during melting?
20. In a sealed container at constant temperature, $\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)}$ reaches a state where the concentrations of all gases remain constant. Which statement best describes the behavior of molecules at equilibrium?

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