The formula of an acidic salt is not a single, universal formula, but rather depends on its specific chemical composition. Acidic salts are compounds that, when dissolved in water, produce an acidic solution. This acidity can arise from two primary mechanisms: either the salt contains a replaceable hydrogen atom, or its cation is derived from a weak base and undergoes hydrolysis.
Understanding Acidic Salts
An acidic salt is essentially a salt that behaves as an acid. This can happen in one of two main ways:
- Presence of Replaceable Hydrogen: The salt retains one or more acidic hydrogen atoms from a polyprotic acid (an acid capable of donating more than one proton). These hydrogen atoms can then be released into solution, making it acidic.
- Cation Hydrolysis: The salt is formed from the reaction of a strong acid and a weak base. The cation of the weak base can react with water (hydrolyze) to produce hydronium ions (H₃O⁺), leading to an acidic solution.
Formulas of Acidic Salts with Replaceable Hydrogen
These salts are formed when a polyprotic acid is only partially neutralized by a base. This means some of the acidic hydrogen atoms of the acid remain in the salt's anionic structure.
General Representation: Mₓ(HyA)z
Where:
- M represents the metal cation (e.g., Na⁺, K⁺, Ca²⁺).
- H represents the remaining acidic hydrogen atom(s).
- A represents the non-hydrogen part of the acid's anion.
- x, y, z are stoichiometric coefficients ensuring charge neutrality.
For example, if you partially neutralize sulfuric acid (H₂SO₄), a diprotic acid, with sodium hydroxide, you can form sodium hydrogen sulfate (NaHSO₄). Here, one acidic hydrogen from H₂SO₄ remains in the anion (HSO₄⁻).
Examples of Acidic Salts with Replaceable Hydrogen
| Name | Chemical Formula | Parent Acid | Parent Base | Acidity Source |
|---|---|---|---|---|
| Sodium Bicarbonate | NaHCO₃ | Carbonic Acid (H₂CO₃) | Sodium Hydroxide | Contains one replaceable H in HCO₃⁻ |
| Sodium Bisulfate | NaHSO₄ | Sulfuric Acid (H₂SO₄) | Sodium Hydroxide | Contains one replaceable H in HSO₄⁻ |
| Sodium Dihydrogen Phosphate | NaH₂PO₄ | Phosphoric Acid (H₃PO₄) | Sodium Hydroxide | Contains two replaceable H in H₂PO₄⁻ |
| Potassium Hydrogen Sulfite | KHSO₃ | Sulfurous Acid (H₂SO₃) | Potassium Hydroxide | Contains one replaceable H in HSO₃⁻ |
Formulas of Acidic Salts from Strong Acid and Weak Base
These salts do not necessarily contain replaceable hydrogen atoms. Instead, their acidity arises from the hydrolysis of their cation when dissolved in water. The cation, being the conjugate acid of a weak base, donates a proton to water molecules, producing hydronium ions.
General Representation: BxAy
Where:
- B represents the cation derived from a weak base (e.g., NH₄⁺, Fe³⁺, Al³⁺).
- A represents the anion derived from a strong acid (e.g., Cl⁻, NO₃⁻, SO₄²⁻).
- x, y are stoichiometric coefficients ensuring charge neutrality.
A classic example is ammonium chloride (NH₄Cl). It is formed from the reaction of ammonia (a weak base) and hydrogen chloride (a strong acid). When ammonium chloride dissolves in water, the ammonium ion (NH₄⁺) hydrolyzes:
NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)
This reaction produces hydronium ions (H₃O⁺), making the solution acidic.
Examples of Acidic Salts from Strong Acid and Weak Base
| Name | Chemical Formula | Parent Acid | Parent Base | Acidity Source |
|---|---|---|---|---|
| Ammonium Chloride | NH₄Cl | Hydrochloric Acid (HCl) | Ammonia (NH₃) | NH₄⁺ cation hydrolyzes to produce H₃O⁺ |
| Iron(III) Chloride | FeCl₃ | Hydrochloric Acid (HCl) | Iron(III) Hydroxide | Fe³⁺ cation hydrolyzes to produce H₃O⁺ |
| Aluminum Sulfate | Al₂(SO₄)₃ | Sulfuric Acid (H₂SO₄) | Aluminum Hydroxide | Al³⁺ cation hydrolyzes to produce H₃O⁺ |
| Copper(II) Nitrate | Cu(NO₃)₂ | Nitric Acid (HNO₃) | Copper(II) Hydroxide | Cu²⁺ cation hydrolyzes to produce H₃O⁺ |
Key Characteristics of Acidic Salt Formulas
- Ionic Compounds: All acidic salts are ionic compounds composed of a cation and an anion.
- Charge Neutrality: The sum of the positive charges from the cations must balance the sum of the negative charges from the anions.
- Origin of Acidity: The formula indicates the potential source of acidity: either an unneutralized hydrogen within the anion (e.g., HSO₄⁻) or a cation derived from a weak base (e.g., NH₄⁺, Fe³⁺) that can hydrolyze.
Understanding these different categories is crucial, as the specific formula of an acidic salt will always reflect its unique ionic composition and the way it generates acidity in solution.
