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BaSO₄ has very low Ksp → least soluble. The others are soluble salts.

CaCO₃ dissolves as Ca²⁺ + CO₃²⁻. Adding acid consumes CO₃²⁻ → HCO₃⁻/H₂CO₃, shifting equilibrium right to dissolve more CaCO₃. ACS tests the link between solubility and acid–base equilibria. Neutral salts like NaCl or KNO₃ are unaffected.

Set up an ICE table in terms of partial pressures. Let x be the dissociation of N₂O₄: P(N₂O₄) = 1.00 − x, P(NO₂) = 2x. The equilibrium constant in pressure terms is Kp = (P_NO₂)² / P_N₂O₄ = (2x)² / (1 − x) = 4x²/(1 − x) = 0.15. Solving gives x ≈ 0.176, so P(NO₂) = 2x ≈ 0.352 atm (choice C). This aligns with ACS equilibrium competencies: translating a chemical equation into an equilibrium expression, constructing an ICE table, and solving a quadratic while checking that the result is physically reasonable (x < initial pressure). Distractors often correspond to using concentration-based Kc without converting, plugging in initial conditions instead of equilibrium values, or neglecting the stoichiometric factor of 2 for NO₂. Because total moles of gas increase upon dissociation, the formation of NO₂ is favored by low total pressure; however, numerical evaluation via Kp provides the exact equilibrium composition, a key quantitative skill on the exam.

Lewis structure analysis gives SF₄ with 5 electron domains around S (4 bonding pairs, 1 lone pair): an AX₄E species. VSEPR predicts a trigonal-bipyramidal electron-domain arrangement with the lone pair in an equatorial site to minimize 90° repulsions. The resulting molecular geometry is see-saw. Bond polarity (S–F) combined with asymmetric shape prevents complete dipole cancellation, so SF₄ is polar. Common pitfalls include confusing electron-domain geometry (trigonal bipyramidal) with molecular geometry (see-saw) or placing the lone pair axially, which would increase repulsion. Recognizing geometry from AXE notation and mapping that to polarity aligns with ACS bonding outcomes, which integrate Lewis structures, formal charge checks, VSEPR, and molecular polarity predictions. Understanding 3D geometry also supports rationalizing molecular properties such as boiling point trends and reactivity patterns, and it connects to hybridization descriptions often used to interpret shapes qualitatively.

Osmotic pressure equation: π = iMRT, where i = van’t Hoff factor. ACS emphasizes colligative property formulas.