Select the specified conjugate base from each of the...

Select the specified conjugate base from each of the following groups of acids: a) Weakest conjugate base of: phosphoric acid, $\mathrm{H}_{3} \mathrm{PO}_{4}$; acetic acid, $\mathrm{CH}_{3} \mathrm{COOH} ;$ hydrogen carbonate ion, $\mathrm{HCO}_{3}^{-}$ b) Strongest conjugate base of: hydrosulfuric acid, $\mathrm{H}_{2} \mathrm{S} ;$ sulfurous acid, $\mathrm{H}_{2} \mathrm{SO}_{3} ;$ hydrocyanic acid, HCN

Select the specified conjugate base from each of the following groups of acids:
a) Weakest conjugate base of: phosphoric acid, $\mathrm{H}_{3} \mathrm{PO}_{4}$; acetic acid, $\mathrm{CH}_{3} \mathrm{COOH} ;$ hydrogen carbonate ion, $\mathrm{HCO}_{3}^{-}$
b) Strongest conjugate base of: hydrosulfuric acid, $\mathrm{H}_{2} \mathrm{S} ;$ sulfurous acid, $\mathrm{H}_{2} \mathrm{SO}_{3} ;$ hydrocyanic acid, HCN

Key Concepts

Stability Factors of Conjugate Bases

The stability of a conjugate base, which can be influenced by factors such as resonance stabilization, inductive effects, and electronegativity, is a determining factor in its basicity. More stable conjugate bases (often resulting from resonance or electron-withdrawing groups) are generally weaker bases because the negative charge is better accommodated, while less stable conjugate bases are stronger bases.

Conjugate Acid?Base Pairs

This concept deals with the relationship between an acid and the species that remains after the acid donates a proton – its conjugate base. In acid–base reactions, every acid has a conjugate base, and the relative strengths of acids and bases are inversely related; the stronger the acid, the weaker its conjugate base, and vice versa.

Acid Strength and pKa

Acid strength is quantified by the acid dissociation constant (Ka) or its logarithmic form (pKa). Lower pKa values indicate stronger acids, which in turn yield weaker conjugate bases because the acid more readily donates its proton. This relationship is key in comparing different acids to determine which will have a stronger or weaker conjugate base.

Equilibrium Principles in Acid–Base Reactions

Acid–base equilibria are established when an acid donates a proton to a base, forming a conjugate base and a conjugate acid. The position of the equilibrium depends on the relative strengths of the acid and base. Understanding this dynamic is essential when predicting the outcome of proton transfer reactions and selecting which conjugate base is stronger or weaker in a set of competing species.

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