Predict the stronger acid in each pair: (b) H3PO4 or H3AsO4 (d) H2C2O4 or HC2O4⁻

Verified step by step guidance

1. To determine the stronger acid between H3PO4 and H3AsO4, consider the position of phosphorus (P) and arsenic (As) in the periodic table. Acidity generally increases with the electronegativity of the central atom. Since P is more electronegative than As, H3PO4 is expected to be the stronger acid.
2. For the pair H2C2O4 and HC2O4⁻, recognize that H2C2O4 is oxalic acid, a diprotic acid, and HC2O4⁻ is its conjugate base after losing one proton. In general, the neutral form of a diprotic acid (H2C2O4) is stronger than its conjugate base (HC2O4⁻) because it can donate a proton more readily.
3. Consider the concept of conjugate acid-base pairs: the stronger the acid, the weaker its conjugate base. Therefore, H2C2O4, being the original acid, is stronger than HC2O4⁻.
4. Review the periodic trends and the concept of electronegativity to understand why H3PO4 is stronger than H3AsO4. Electronegativity affects the ability of the central atom to stabilize the negative charge after deprotonation.
5. Understand that the ability of a molecule to donate protons (H⁺ ions) is a key factor in determining its acidity. The more readily a molecule can donate a proton, the stronger the acid it is.

Key Concepts

Acid Strength

Acid strength refers to the ability of an acid to donate protons (H⁺ ions) in a solution. Stronger acids dissociate more completely in water, resulting in a higher concentration of H⁺ ions. The strength of an acid can be influenced by factors such as the stability of the conjugate base formed after deprotonation and the electronegativity of the atoms involved.

Conjugate Base Stability

The stability of a conjugate base is crucial in determining the strength of its corresponding acid. A more stable conjugate base will favor the dissociation of the acid, making it stronger. Factors affecting stability include resonance, electronegativity, and the size of the atom bearing the negative charge. For example, if a conjugate base can delocalize its negative charge through resonance, it will be more stable.

Comparative Acid-Base Properties

When comparing acids, it is essential to consider their molecular structure and the elements involved. For instance, acids containing central atoms from different groups in the periodic table may exhibit varying strengths due to differences in electronegativity and atomic size. Additionally, the presence of electron-withdrawing or electron-donating groups can significantly influence the acid's ability to donate protons.