Understanding Diprotic Acids

A diprotic acid is an acid that can donate two protons (H⁺ ions) per molecule in aqueous solution. To identify them, we examine the number of acidic hydrogen atoms in each compound.

Step 1: List and Analyze Each Acid

H₃PO₄ (Phosphoric acid): Has three acidic hydrogens, but is triprotic, not diprotic.

H₂SO₄ (Sulfuric acid): Has two acidic hydrogens, so it is diprotic.

H₃PO₃ (Phosphorous acid): Has two acidic hydrogens (one H is bonded to P, not acidic), so it is diprotic.

H₂CO₃ (Carbonic acid): Has two acidic hydrogens, so it is diprotic.

H₂S₂O₇ (Disulfuric acid): Has two acidic hydrogens, so it is diprotic.

H₃BO₃ (Boric acid): Acts as a monoprotic acid (donates one H⁺), not diprotic.

H₃PO₂ (Hypophosphorous acid): Has one acidic hydrogen, so it is monoprotic.

H₂CrO₄ (Chromic acid): Has two acidic hydrogens, so it is diprotic.

H₂SO₃ (Sulfurous acid): Has two acidic hydrogens, so it is diprotic.

Step 2: Count the Diprotic Acids

From the list, the diprotic acids are: H₂SO₄, H₃PO₃, H₂CO₃, H₂S₂O₇, H₂CrO₄, H₂SO₃.

That gives a total of 6 diprotic acids.

Final Answer

The total number of diprotic acids is $6$.

Related Topics

Acid Strength: Depends on the ability to donate protons; stronger acids have lower pKa values.

Polyprotic Acids: Acids that can donate more than one proton, such as diprotic (2 H⁺), triprotic (3 H⁺), etc.

Acidic Hydrogen: Hydrogen atoms attached to electronegative atoms (like O) that can be released as H⁺ ions.

Key Formulae and Theory

For a diprotic acid H₂A, the dissociation occurs in two steps:

First dissociation: $H_{2}A\rightleftharpoons H^{+}+{\mathrm{HA}}^{-}$, with $K_{a1}$

Second dissociation: ${\mathrm{HA}}^{-}\rightleftharpoons H^{+}+A^{2-}$, with $K_{a2}$

Generally, $K_{a1}>K_{a2}$ due to the negative charge on HA⁻ making further dissociation harder.