Understanding Tribasic Acids

Acids are substances that can donate hydrogen ions ($\text{H}^+$) when dissolved in water. The number of hydrogen ions that an acid molecule can donate is called its basicity or proticity. Based on basicity, acids can be classified as monobasic, dibasic, or tribasic.

What is a Tribasic Acid?

A tribasic acid is an acid that can donate three hydrogen ions ($\text{H}^+$) per molecule when it dissociates in an aqueous solution. This means it undergoes three steps of dissociation.

Analyzing the Given Options

Let's examine the basicity of each acid provided in the options:

1. Hydrochloric acid ($\text{HCl}$): This acid has one ionizable hydrogen atom. It dissociates in one step:
   $\text{HCl} \text{(aq)} \rightarrow \text{H}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$
   It can donate only one $\text{H}^+$ ion. Therefore, hydrochloric acid is a monobasic acid.
2. Nitric acid ($\text{HNO}_3$): This acid also has one ionizable hydrogen atom. Its dissociation is:
   $\text{HNO}_3 \text{(aq)} \rightarrow \text{H}^+ \text{(aq)} + \text{NO}_3^- \text{(aq)}$
   It can donate only one $\text{H}^+$ ion. Therefore, nitric acid is a monobasic acid.
3. Sulphuric acid ($\text{H}_2\text{SO}_4$): This acid has two ionizable hydrogen atoms. It dissociates in two steps:
   $\text{H}_2\text{SO}_4 \text{(aq)} \rightarrow \text{H}^+ \text{(aq)} + \text{HSO}_4^- \text{(aq)}$
   $\text{HSO}_4^- \text{(aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{SO}_4^{2-} \text{(aq)}$
   It can donate up to two $\text{H}^+$ ions. Therefore, sulphuric acid is a dibasic acid.
4. Phosphoric acid ($\text{H}_3\text{PO}_4$): This acid has three ionizable hydrogen atoms attached to oxygen atoms. It dissociates in three steps:
   $\text{H}_3\text{PO}_4 \text{(aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{H}_2\text{PO}_4^- \text{(aq)}$
   $\text{H}_2\text{PO}_4^- \text{(aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{HPO}_4^{2-} \text{(aq)}$
   $\text{HPO}_4^{2-} \text{(aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{PO}_4^{3-} \text{(aq)}$
   It can donate up to three $\text{H}^+$ ions. Therefore, phosphoric acid is a tribasic acid.

Summary of Acids and Their Basicity

From the analysis, Phosphoric acid ($\text{H}_3\text{PO}_4$) is the only acid among the options that can donate three hydrogen ions, making it a tribasic acid.

Revision Table: Acid Basicity Concepts

Additional Information on Polyprotic Acids

Acids that can donate more than one proton (like dibasic and tribasic acids) are called polyprotic acids. The dissociation of polyprotic acids occurs in steps, and each step has its own dissociation constant ($\text{K}_a$).

• For a diprotic acid ($\text{H}_2\text{A}$):
  1. $\text{H}_2\text{A} \rightleftharpoons \text{H}^+ + \text{HA}^-$ ($\text{K}_{a1}$)
  2. $\text{HA}^- \rightleftharpoons \text{H}^+ + \text{A}^{2-}$ ($\text{K}_{a2}$)
  Typically, $\text{K}_{a1} > \text{K}_{a2}$. The first proton is easier to remove than the second one.
• For a triprotic acid ($\text{H}_3\text{A}$):
  1. $\text{H}_3\text{A} \rightleftharpoons \text{H}^+ + \text{H}_2\text{A}^-$ ($\text{K}_{a1}$)
  2. $\text{H}_2\text{A}^- \rightleftharpoons \text{H}^+ + \text{HA}^{2-}$ ($\text{K}_{a2}$)
  3. $\text{HA}^{2-} \rightleftharpoons \text{H}^+ + \text{A}^{3-}$ ($\text{K}_{a3}$)
  Typically, $\text{K}_{a1} > \text{K}_{a2} > \text{K}_{a3}$. Removing subsequent protons becomes increasingly difficult due to the increasing negative charge on the species.

Phosphoric acid ($\text{H}_3\text{PO}_4$) is a common example of a triprotic acid, widely used in fertilizers, detergents, and food products.