18. The unit cell of the perovskite is shown below. The lattice points consist of some A, B cations, and oxygen anions, whose radii are denoted as rA, rB, and ro, respectively. Based on its structure, choose the INCORRECT statement. (A) The formula is ABO₃ (B) There is one octahedral hole in this unit cell (C) The length of the unit cell is $2r_{\mathrm{A}}$ (D) The unit cell can be expressed in another form (E) A and B could be Ba and Ti, respectively.

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) $3.0 \times 10^{-3}$

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