41. Of the vibrational normal modes of CO₂ shown on right, which are infrared active? (A) I and II only (B) I and II only (C) II and III only (D) I, II, and III (E) None

4. Both ZnS and CaF₂ have a face-centered cubic unit cell where the S²⁻ and Ca²⁺ ions are closest packed in each structure. Which of the following is true? (A) There are 4 tetrahedral holes empty in each structure. (B) In both compounds, one-half of the tetrahedral holes are filled. (C) In both compounds, all the tetrahedral holes are filled. (D) There are 8 Zn²⁺ ions and 4 F⁻ ions in the unit cell. (E) In ZnS, one-half of the tetrahedral holes are filled by Zn²⁺ ions, whereas in CaF₂ all the tetrahedral holes are filled with F⁻ ions.

41. Of the vibrational normal modes of CO₂ shown on right, which are infrared active?

(A) I and II only
(B) I and II only
(C) II and III only
(D) I, II, and III
(E) None

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