Question

Based on their compositions and structures and on conjugate acid-base relationships, select the stronger base in each of the following pairs: (a) \(\mathrm{BrO}^{-}\) or \(\mathrm{ClO}^{-}\), (b) \(\mathrm{BrO}^{-}\) or \(\mathrm{BrO}_{2}^{-},(\mathrm{c}) \mathrm{HPO}_{4}^{2-}\) or \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)

Short answer

The stronger bases for each pair are: (a) BrO⁻, (b) BrO⁻, and (c) HPO₄²⁻.

Step-by-step solution

(a) Comparing BrO⁻ and ClO⁻

First, let's identify the conjugate acids of these two bases: BrOH for BrO⁻ and ClOH for ClO⁻. In general, the more electronegative the element, the more stable the conjugate acid will be. Here, Cl is more electronegative than Br, which means that the conjugate acid ClOH is stronger than BrOH. Since the stronger the conjugate acid, the weaker the conjugate base, the stronger base in this case is BrO⁻.

(b) Comparing BrO⁻ and BrO₂⁻

First, we need to identify the conjugate acids: BrOH for BrO⁻ and BrO₂H for BrO₂⁻. Both bases have Br as the central atom, but BrO₂⁻ has more oxygen atoms than BrO⁻. Generally, the presence of more oxygen atoms increases the stability of the conjugate acid due to the electron-donating effect of oxygen atoms, which can help dissipate the charge. Since the conjugate acid of BrO₂⁻ is more stable than the conjugate acid of BrO⁻, BrO₂⁻ is the weaker base. Therefore, BrO⁻ is the stronger base.

(c) Comparing HPO₄²⁻ and H₂PO₄⁻

The conjugate acids for these bases are H₂PO₄⁻ for HPO₄²⁻ and H₃PO₄ for H₂PO₄⁻. We can see that H₃PO₄ has more hydrogen atoms (meaning it is more acidic) compared to H₂PO₄⁻, which indicates a stronger conjugate acid. Since the stronger the conjugate acid, the weaker the conjugate base, H₂PO₄⁻ would be a weaker base compared to HPO₄²⁻. Thus, the stronger base is HPO₄²⁻.