Based on their compositions and structures and on conjugate...

Based on their compositions and structures and on conjugate acid-base relationships, select the stronger base in each of the following pairs: (a) $\mathrm{NO}_{3}^{-}$ or $\mathrm{NO}_{2}^{-},$ (b) $\mathrm{PO}_{4}^{3-}$ or $\mathrm{AsO}_{4}^{3-}$, (c) $\mathrm{HCO}_{3}^{-}$ or $\mathrm{CO}_{3}^{2-}$.

Based on their compositions and structures and on conjugate acid-base relationships, select the stronger base in each of the following pairs:
(a) $\mathrm{NO}_{3}^{-}$ or $\mathrm{NO}_{2}^{-},$ (b) $\mathrm{PO}_{4}^{3-}$ or $\mathrm{AsO}_{4}^{3-}$,
(c) $\mathrm{HCO}_{3}^{-}$ or $\mathrm{CO}_{3}^{2-}$.

Key Concepts

Conjugate Acid?Base Relationships

This concept describes the inverse relationship between the strength of an acid and its conjugate base. A strong acid produces a weak conjugate base, and vice versa, which is crucial when comparing bases because the pKa values of their conjugate acids can provide quantitative insight into their relative basicities.

Resonance Stabilization

Resonance involves the delocalization of electrons across multiple atoms, stabilizing an ion. When a negative charge is distributed over several atoms via resonance, the ion is more stabilized and tends to be less reactive as a base. Thus, molecules with less opportunity for resonance stabilization generally exhibit higher basicity.

Periodic Trends in Electronegativity and Atomic Size

Electronegativity and atomic size affect the ability of ions to stabilize or destabilize negative charges. More electronegative atoms more effectively stabilize negative charge, which can decrease basicity. Similarly, larger atoms have more diffuse charge distributions, which influences how the base interacts with protons. These periodic trends help explain why bases with different central atoms (or substituents) can exhibit different strengths.

Charge Density and Its Influence on Basicity

Charge density refers to the concentration of negative charge in an ion. Ions with higher charge density, meaning more negative charge packed into a smaller volume, are generally more reactive as bases because they are less stabilized. Comparing ions like bicarbonate and carbonate highlights how an extra unit of negative charge can enhance basicity, assuming the structure does not excessively delocalize or stabilize the charge.

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