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Alpha decay reduces atomic number by 2 and mass number by 4. U-238 (Z=92) → Th-234 (Z=90). ACS objectives stress nuclear decay equations and conservation laws. This concept is key in nuclear physics, radiometric dating, and nuclear energy cycles.

Rate ∝ [A]ⁿ. If doubling [A] increases rate by a factor of 4, then n = 2. Thus, the reaction is second order with respect to that reactant. ACS objectives stress determining reaction order from experimental data and applying rate laws.

ΔG = ΔH – TΔS. With ΔH negative and ΔS negative, spontaneity occurs at low T where the enthalpy term dominates. At high T, –TΔS is positive and may make ΔG positive. ACS objectives stress applying Gibbs free energy to predict spontaneity across conditions.

Molar mass NH₃ = 14 + (3 × 1) = 17 g/mol.

%N = 14 ÷ 17 × 100 = 82%.

ACS objectives stress percent composition analysis, a key technique in empirical formula determination and quality control in chemistry.

Water exhibits extensive hydrogen bonding, requiring significant energy to vaporize. H₂S and CH₄ lack hydrogen bonding, while NH₃ has weaker hydrogen bonding than H₂O. ACS objectives emphasize how intermolecular forces affect enthalpy changes. This explains water’s unique physical properties and its role in climate regulation and biological systems.