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[Ne] represents the first 10 electrons. Adding 3s² and 3p² accounts for 4 more, totaling 14 electrons. The element with atomic number 14 is silicon (Si). Silicon is a metalloid and essential in semiconductors. Magnesium has [Ne]3s², phosphorus is [Ne]3s²3p³, and sulfur is [Ne]3s²3p⁴. ACS objectives require mastery of electron configurations and their connection to periodic properties. This understanding explains trends in reactivity, metallic vs nonmetallic behavior, and chemical bonding patterns.

Na₂SO₄ dissociates completely in water: Na₂SO₄ → 2Na⁺ + SO₄²⁻.

Each mole of Na₂SO₄ yields 1 mole of SO₄²⁻. Thus, a 0.020 M solution produces 0.020 M SO₄²⁻. Wait—but check carefully: For every 1 mol Na₂SO₄, there are 2 mol Na⁺ and 1 mol SO₄²⁻. Therefore, [SO₄²⁻] = 0.020 M, not 0.040 M. Correct answer is B.

This demonstrates how dissociation stoichiometry must be carefully applied in ion concentration calculations. ACS objectives highlight calculating ion concentrations, applying solubility rules, and predicting precipitation in double displacement reactions.

Molecules = mol × Avogadro’s number = 0.75 × 6.022 × 10²³ = 4.52 × 10²³. Correct answer is B.

ACS objectives stress mole–particle conversions using Avogadro’s number, central in stoichiometry and chemical analysis.

Adding reactant (O₂) shifts equilibrium toward products (SO₃). Because the forward reaction is exothermic, lowering temperature also favors SO₃. ACS objectives stress Le Châtelier’s Principle, applied to industrial processes like sulfuric acid production.

Equilibrium is dynamic, with forward and reverse rates equal, so concentrations remain constant but not necessarily equal. ΔG = 0 at equilibrium. ACS objectives stress equilibrium constants, Le Châtelier’s principle, and free energy relationships.