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s orbitals are spherical. p = dumbbell, d = clover, f = complex. ACS emphasizes orbital shapes.

Balanced: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Moles H₂SO₄ = 0.0500 L × 0.100 M = 0.00500 mol. Requires 2 × 0.00500 = 0.0100 mol NaOH. With 0.200 M NaOH: V = n/M = 0.0100/0.200 = 0.0500 L = 50.0 mL. Correct answer is C. Careful step-check: matches. ACS emphasizes setting up neutralization stoichiometry with correct coefficients. Errors often involve forgetting the diprotic nature of H₂SO₄. This connects acid–base chemistry to volumetric analysis.

Precipitation occurs when [M²⁺][SO₄²⁻] exceeds Ksp. Threshold [SO₄²⁻] = Ksp/[M²⁺]. For BaSO₄: (1.1 × 10⁻¹⁰)/(0.010) = 1.1 × 10⁻⁸ M. For CaSO₄: (2.4 × 10⁻⁵)/(0.010) = 2.4 × 10⁻³ M. Since much less sulfate is needed for BaSO₄ to precipitate, BaSO₄ precipitates first. ACS tests selective precipitation, comparing thresholds systematically. A typical mistake is assuming higher Ksp precipitates first; in fact, the lower threshold concentration indicates earlier precipitation. This principle underlies qualitative analysis schemes.

H₂O can act as both an acid (donating H⁺) and a base (accepting H⁺). ACS emphasizes amphoteric behavior.

NaCl dissociates into ions, lowering solvent vapor pressure, raising boiling point (colligative property).