Based on their compositions and structures and on conjugate acid–base relationships, select the stronger base in each of the following pairs: (b) PO43- or AsO43-

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Identify the conjugate acids of the given bases: HPO4^{2-} for PO4^{3-} and HAsO4^{2-} for AsO4^{3-}.

Compare the acid strengths of HPO4^{2-} and HAsO4^{2-}. The weaker the conjugate acid, the stronger the base.

Consider the position of phosphorus (P) and arsenic (As) in the periodic table. Elements in the same group often have similar properties, but acidity can vary based on electronegativity and atomic size.

Phosphorus is above arsenic in the periodic table, which generally means HPO4^{2-} is a weaker acid than HAsO4^{2-} due to the smaller size and higher electronegativity of phosphorus.

Conclude that PO4^{3-} is the stronger base because its conjugate acid, HPO4^{2-}, is weaker than the conjugate acid of AsO4^{3-}.

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Conjugate Acid-Base Pairs

Conjugate acid-base pairs consist of two species that differ by the presence of a proton (H+). When an acid donates a proton, it forms its conjugate base, while a base that accepts a proton forms its conjugate acid. Understanding these relationships is crucial for determining the strength of acids and bases, as stronger bases have weaker conjugate acids.

Base Strength and Stability

The strength of a base is influenced by its ability to accept protons and the stability of the resulting conjugate acid. A more stable conjugate acid indicates a stronger base, as it is less likely to donate a proton back. Factors affecting stability include electronegativity, resonance, and the size of the atom holding the negative charge.

Comparative Analysis of Anions

When comparing anions like PO4^3- and AsO4^3-, it is essential to consider their atomic structure and electronegativity. The central atom's size and electronegativity affect the anion's ability to stabilize the negative charge. Generally, larger atoms can better accommodate negative charges, influencing the relative strength of the bases formed by these anions.