Label each of the following as being a strong acid, a weak acid, or a species with negligible acidity. In each case write the formula of its conjugate base, and indicate whether the conjugate base is a strong base, a weak base, or a species with negligible basicity: (a) \(\mathrm{HCOOH},\) (b) \(\mathrm{H}_{2},\) (c) \(\mathrm{CH}_{4}\), (d) \(\mathrm{HF}\) (e) \(\mathrm{NH}_{4}^{+}\).

Understanding conjugate acid-base pairs is fundamental in acid-base chemistry. A conjugate acid-base pair consists of two substances that transform into each other by the gain or loss of a proton (a hydrogen ion, \( \mathrm{H}^{+} \)). When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

For instance, in the case of formic acid (\( \mathrm{HCOOH} \)), when it donates a proton, the conjugate base formed is the formate ion (\( \mathrm{HCOO}^{-} \)). The acid and base are conjugates of one another. Similarly, for the ammonium ion (\( \mathrm{NH}_{4}^{+} \)), when it donates a proton, ammonia (\( \mathrm{NH}_{3} \)) is the resulting conjugate base.

It's important to recognize that the strength of a conjugate acid is inversely related to the strength of its conjugate base. A strong acid will have a weak conjugate base, while a weak acid will have a weak conjugate base that is stronger than that of a strong acid. Species with negligible acidity, like methane (\( \mathrm{CH}_{4} \)), have conjugate bases (methyl anion, \(( \mathrm{CH}_{3}^{-} \) with negligible basicity.

The classification of weak acids and bases is central to predicting the behavior of substances in a variety of chemical contexts. A weak acid, such as formic acid (\( \mathrm{HCOOH} \)) or hydrofluoric acid (\( \mathrm{HF} \)), does not completely dissociate in water; it only partially ionizes, releasing some \( \mathrm{H}^{+} \) ions and leaving an equilibrium between the undissociated acid and its ions.

On the other hand, weak bases, like fluoride ion (\( \mathrm{F}^{-} \)) and ammonia (\( \mathrm{NH}_{3} \)), only partially accept protons and do not fully ionize in solution. Their tendency to accept protons is not as strong as that of strong bases, which fully ionize and accept protons readily.

Knowledge of weak acid and base behavior is crucial when solving problems in acid-base reactions, making predictions on reaction outcomes, and calculating pH levels in solutions. For example, when hydrofluoric acid (a weak acid) is dissolved in water, it creates a dynamic equilibrium with its conjugate base, the fluoride ion (a weak base), partially accepting protons from the water molecules.

Some substances exhibit negligible acidity and basicity. These are species that neither donate protons readily nor accept them easily. Negligible acidity is often seen in substances that are essentially non-acidic, such as methane (\( \mathrm{CH}_{4} \)) and hydrogen gas (\( \mathrm{H}_{2} \)). Because these substances don't donate protons, they do not have a tendency to behave as acids in water.

Similarly, species like the methyl anion (\( \mathrm{CH}_{3}^{-} \)), which are the conjugate bases of negligibly acidic molecules, also exhibit negligible basicity; they have a minimal tendency to accept protons. These properties affect their reactivity and behavior in chemical reactions. Understanding these concepts is important when determining the potential for a substance to participate in acid-base chemistry and predicting the direction of a reaction. In most cases, species with negligible acidity or basicity won't significantly alter the pH of a solution.