Count Total Number of Diprotic Acids From Given List

To determine the total number of diprotic acids, we evaluate the proticity (basicity) of each given acid based on its chemical structure and ionization behavior in aqueous solution.

A diprotic (or dibasic) acid is an acid that yields two hydrogen ions ($\text{H}^+$) per molecule upon ionization in aqueous solution.

Let us analyze each given species:

1. $\text{H}_3\text{PO}_4$ (Phosphoric acid):

   • Structure: $\text{O}=\text{P}(\text{OH})_3$
   • It contains three acidic $-\text{OH}$ protons bonded to the phosphorus atom.
   • Basicity = $3$ (Triprotic)

2. $\text{H}_2\text{SO}_4$ (Sulfuric acid):

   • Structure: $\text{O}_2\text{S}(\text{OH})_2$
   • It contains two acidic $-\text{OH}$ protons bonded to the sulfur atom.
   • Basicity = $2$ (Diprotic)

3. $\text{H}_3\text{PO}_3$ (Phosphorous acid):

   • Structure: $\text{O}=\text{P}(\text{H})(\text{OH})_2$
   • It contains two acidic $-\text{OH}$ groups and one non-ionizable $\text{P}-\text{H}$ bond.
   • Basicity = $2$ (Diprotic)

4. $\text{H}_2\text{CO}_3$ (Carbonic acid):

   • Structure: $\text{O}=\text{C}(\text{OH})_2$
   • It contains two acidic $-\text{OH}$ protons bonded to the carbon atom.
   • Basicity = $2$ (Diprotic)

5. $\text{H}_2\text{S}_2\text{O}_7$ (Pyrosulfuric acid / Oleum):

   • Structure: $\text{HO}-\text{SO}_2-\text{O}-\text{SO}_2-\text{OH}$
   • It contains two acidic $-\text{OH}$ protons.
   • Basicity = $2$ (Diprotic)

6. $\text{H}_3\text{BO}_3$ (Boric acid):

   • It does not ionize by donating its own protons; instead, it acts as a monobasic Lewis acid by accepting an $\text{OH}^-$ ion from water:
     $\text{B}(\text{OH})_3 + \text{H}_2\text{O} \rightleftharpoons [\text{B}(\text{OH})_4]^- + \text{H}^+$
   • Basicity = $1$ (Monoprotic)

7. $\text{H}_3\text{PO}_2$ (Hypophosphorous acid):

   • Structure: $\text{O}=\text{P}(\text{H})_2(\text{OH})$
   • It contains only one acidic $-\text{OH}$ proton and two non-ionizable $\text{P}-\text{H}$ bonds.
   • Basicity = $1$ (Monoprotic)

8. $\text{H}_2\text{CrO}_4$ (Chromic acid):

   • Structure: $\text{O}_2\text{Cr}(\text{OH})_2$
   • It contains two acidic $-\text{OH}$ protons bonded to the chromium atom.
   • Basicity = $2$ (Diprotic)

9. $\text{H}_2\text{SO}_3$ (Sulfurous acid):

   • Structure: $\text{O}=\text{S}(\text{OH})_2$
   • It contains two acidic $-\text{OH}$ protons bonded to the sulfur atom.
   • Basicity = $2$ (Diprotic)

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Conclusion

The diprotic acids among the given list are:
$\text{H}_2\text{SO}_4, \quad \text{H}_3\text{PO}_3, \quad \text{H}_2\text{CO}_3, \quad \text{H}_2\text{S}_2\text{O}_7, \quad \text{H}_2\text{CrO}_4, \quad \text{H}_2\text{SO}_3$

Thus, the total number of diprotic acids is $6$.