20. An aqueous solution was prepared by dissolving 0.05 moles of aluminum sulfate in 1.00 kg of water. The freezing point of this solution is determined to be $-0.391^{\circ}\mathrm{C}$. (Kf for water is $1.86^{\circ}\mathrm{C} \cdot \mathrm{kg} / \mathrm{mol}$). Calculate the van't Hoff factor (i) for aluminum sulfate.
(A) 5
(B) 4.2
(C) 3.6
(D) 2.1
(E) 1 

21. At $100^{\circ}\mathrm{C}$ and 1 atm, $100\mathrm{g}$ sodium azide (hint: a compound with a chemical formula of $\mathrm{NaN_7}$) was completely inflated, what is the volume (liters) of the nitrogen gas released from this sample? (A) >121 (B) 101-120 (C) 81-100 (D) 61-80 (E) 41-60 

22. Cerium(IV) ion is a strong oxidizing agent that accepts one electron to produce cerium(III) ion. A solution containing an unknown concentration of $Sn^{2+}$ ions was titrated with a solution containing $Ce^{4+}$ ions, which oxidize the $Sn^{2+}$ ions to $Sn^{4+}$ ions. In one titration $1.00\ \mathrm{L}$ of the unknown solution required $46.45\ \mathrm{mL}$ of a $0.1050\ \mathrm{M}\ Ce^{4+}$ solution to reach the stoichiometric point. $$ a\ Ce^{4+}(aq) + b\ Sn^{2+}(aq) \longrightarrow c\ Ce^{3+}(aq) + d\ Sn^{4+}(aq) $$ What is the closest concentration of $Sn^{2+}$ ions in the unknown solution? (A) $2.4 \times 10^{-3}$ (B) $2.4 \times 10^{-2}$ (C) $2.7 \times 10^{-3}$ (D) $2.7 \times 10^{-2}$ (E) $3.0 \times 10^{-3}$

23. For a $\mathrm{H}_2\mathrm{CO}_3$ solution at $25^{\circ}\mathrm{C}$, what is the fractions (in %) of HCO3- at pH 9.00?  H2CO3(aq)⇌H+(aq)+HCO3-(aq) Ka1=4.3×10-7 HCO3-(aq) ⇌H+(ag)+CO3-(aq)   Ka2 =4.8 x 10-11  (A) < 20 (B) 20-39 (C) 40-69 (D) 70-90 (E) >90

24. If $\mathrm{H}_3\mathrm{A}$ is a triprotic acid with $k_{a1} = 1 \times 10^{-4}$, $k_{a2} = 1 \times 10^{-8}$, $k_{a3} = 1 \times 10^{-14}$. What is the equilibrium concentrations of $[\mathrm{HA}^{2-}]$ in a $1.0\ \mathrm{M}\ \mathrm{H}_3\mathrm{A}$ aqueous solution? (A) $1 \times 10^{-7}$ (B) $1 \times 10^{-6}$ (C) $1 \times 10^{-10}$ (D) $1 \times 10^{-9}$ (E) $1 \times 10^{-8}$

25. A 50.0-mL sample of $0.10\ \mathrm{M}\ \mathrm{HCN}$ (if $K_a = 7 \times 10^{-10}$) is titrated with $0.10\ \mathrm{M}\ \mathrm{NaOH}$, what is the pH of the solution at the halfway point in the titration? (Select the closest pH) (A) $\sim 9$ (B) $\sim 10$ (C) $\sim 11$ (D) $\sim 12$ (E) $\sim 8$

26. What is the pH of a $1.0\ \mathrm{M}$ solution of $\mathrm{NaH_2PO_4}$? (For $\mathrm{H_3PO_4}$, $K_{a1} = 7.5 \times 10^{-3}$, $K_{a2} = 6.2 \times 10^{-8}$, and $K_{a3} = 4.8 \times 10^{-13}$). Select the closest pH. (A) $\sim 6$ (B) $\sim 5.5$ (C) $\sim 4$ (D) $\sim 4.5$ (E) $\sim 5$

27. What is the solubility (M) of solid AgCI in a 10.0 M NHs solution? AgCI(s)+2NH3(aq) ⇌Ag(NH3)2+ (aq)+CI -(aq) If K=2.8 X 10-3 Select the closest solubility (M). (A)~0.3 (B)~0.4 (C)〜0.5 (D)~0.6 (E)~0.7

28. For a reaction: A $\longrightarrow$ B(g) + C(g) was studied at two temperature and the following values of k were obtained. (Hint: Arrhenius equation: $ = A e?/RT). What is the value of Ea for this reaction? Select the closest Ea (in kJ/mol). (A) ~80 (B) ~100 (C) ~120 (D) ~140 (E) ~160

29. Select the right configuration assignment ($R$ or $S$) for the following two organic compounds.  (A) Compound I is S and compound II is R (B) Compound I is R and compound II is S (C) Compound I is R and compound II is R (D) Compound I is S and compound II is S (E) Can not determine

30. What is the best structure for $\mathbf{C_9H_8O_2}$ which with the following NMR data? $^{1}\mathrm{H}$ NMR data.: $\delta$ 9.89 (1 H, s), 7.88 (2 H, d, $J = 7\mathrm{Hz}$), 7.29 (2 H, d, $J = 7\mathrm{Hz}$), 3.83 (1 H, t, $J = 7\mathrm{Hz}$), 2.96 (1 H, dd, J = 7 & $3.5\mathrm{Hz}$), 2.71 (1 H, dd, J = 7 & $3.5\mathrm{Hz}$). $^{13}\mathrm{C}$ NMR also attached, as shown below. (A)(B)(C)(D)(E)

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