Question

Give the formula for the conjugate base of (a) \(\mathrm{H}_{2} \mathrm{AsO}_{4}^{-}\) (b) \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}(\mathrm{OH})^{+}\) (c) \(\mathrm{HClO}_{3}\) (d) \(\mathrm{NH}_{4}^{+}\) (e) \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

Short answer

Answer: The conjugate bases for the given acids are as follows: (a) \(\mathrm{HAsO}_{4}^{2-}\), (b) \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{O}\), (c) \(\mathrm{ClO}_{3}^{-}\), (d) \(\mathrm{NH}_{3}\), and (e) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\).

Step-by-step solution

(a) Conjugate base of \(\mathrm{H}_{2} \mathrm{AsO}_{4}^{-}\)

To find the conjugate base of \(\mathrm{H}_{2} \mathrm{AsO}_{4}^{-}\), we need to remove a proton (H+) from the acid. After that, the remaining species will have one less hydrogen and one more negative charge: \(\mathrm{H}_{1} \mathrm{AsO}_{4}^{2-}\). Therefore, the conjugate base of \(\mathrm{H}_{2} \mathrm{AsO}_{4}^{-}\) is \(\mathrm{HAsO}_{4}^{2-}\).

(b) Conjugate base of \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}(\mathrm{OH})^{+}\)

To find the conjugate base of \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}(\mathrm{OH})^{+}\), we remove a proton (H+) from the \(\mathrm{OH}\) group. The remaining species will have one less hydrogen and no charge: \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{O}\). Therefore, the conjugate base of \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}(\mathrm{OH})^{+}\) is \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{O}\).

(c) Conjugate base of \(\mathrm{HClO}_{3}\)

To find the conjugate base of \(\mathrm{HClO}_{3}\), we need to remove a proton (H+) from the acid. After that, the remaining species will have one less hydrogen and one more negative charge: \(\mathrm{ClO}_{3}^{-}\). Therefore, the conjugate base of \(\mathrm{HClO}_{3}\) is \(\mathrm{ClO}_{3}^{-}\).

(d) Conjugate base of \(\mathrm{NH}_{4}^{+}\)

To find the conjugate base of \(\mathrm{NH}_{4}^{+}\), we need to remove a proton (H+) from the acid. After that, the remaining species will have one less hydrogen and no charge: \(\mathrm{NH}_{3}\). Therefore, the conjugate base of \(\mathrm{NH}_{4}^{+}\) is \(\mathrm{NH}_{3}\).

(e) Conjugate base of \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

To find the conjugate base of \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\), we need to remove a proton (H+) from the acid. After that, the remaining species will have one less hydrogen and one more negative charge: \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\). Therefore, the conjugate base of \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) is \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\).

Key concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes where an acid and a base react to form a salt and, typically, water. These reactions involve the exchange of protons (H+ ions) between the reactants. When an acid donates a proton, it forms its conjugate base, and when a base accepts a proton, it forms its conjugate acid. This concept is crucial for understanding the nature of acids and bases, and how they transform in chemical reactions.

An example of an acid-base reaction is the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) to form sodium chloride (NaCl) and water (H2O). The process can be thought of as HCl donating a proton to NaOH, resulting in the formation of H2O and the conjugate base Cl-.

Proton Transfer

The core of acid-base chemistry lies in the transfer of protons—a process referred to as proton transfer. The transfer of a proton is the simplest type of acid-base reaction and is the defining feature in the Brønsted-Lowry theory. In a proton transfer, the acid loses a hydrogen ion and becomes a conjugate base, while the base gains the hydrogen ion and becomes a conjugate acid.

Understanding the direction in which proton transfer occurs can help predict the outcome of reactions and the strengths of the acids and bases involved. For instance, stronger acids transfer protons more readily compared to weaker acids. Proton transfer is a reversible process and occurs in both directions, leading to an equilibrium state in most scenarios.

Chemical Formula

A chemical formula is a symbolic representation of the elemental composition of a chemical compound. Understanding the chemical formula is essential for identifying the substance and predicting its properties and behavior in chemical reactions. A formula indicates the types and numbers of atoms present in the simplest ratio. For example, H2O has two atoms of hydrogen for every one atom of oxygen.

Interpreting Chemical Formulas

When analyzing acid-base reactions and their respective conjugate pairs, interpreting the chemical formula enables us to understand changes in composition. By observing the alteration of a chemical formula when it loses or gains a proton, students can identify the change from an acid to its conjugate base and vice versa.

Brønsted-Lowry Theory

The Brønsted-Lowry theory provides a broader understanding of acid-base reactions by focusing on proton donation and acceptance. According to this theory, an acid is a substance that can donate a proton (H+), whereas a base is any substance that can accept a proton. This theory is particularly helpful for understanding reactions in aqueous solutions as it moves beyond the concept of acid and bases being defined by their ability to produce H+ or OH- ions.

The Brønsted-Lowry theory extends the range of compounds that can be classified as acids or bases. For example, while NH4+ may not seem like an acid in the classical sense, it does act as a Brønsted-Lowry acid because it can donate a proton to form NH3. The corresponding conjugate base, NH3, can further accept a proton in a different reaction, exemplifying the reversible nature of these interactions.