Problem 175 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of these is not an acid salt ? (a) \(\mathrm{NaH}_{2} \mathrm{PO}_{2}\) (b) \(\mathrm{NaH}_{2} \mathrm{PO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\)

Step-by-Step Solution

Answer

The compound \(\mathrm{NaH}_2 \mathrm{PO}_2\) is not an acid salt.

Step 1: Understanding Acid Salts

An acid salt is a salt containing hydrogen ions (H\(^+\)), which are replaceable when in solution. It typically forms when a polybasic acid is not completely neutralized.

Step 2: Examining Option (a)

- Compound: \(\mathrm{NaH}_2 \mathrm{PO}_2\). - Hypophosphorous acid (H\(_3\)PO\(_2\)) forms \(\mathrm{NaH}_2\mathrm{PO}_2\) when partially neutralized. - Only has one replaceable H; forms only normal salt, not an acid salt.

Step 3: Examining Option (b)

- Compound: \(\mathrm{NaH}_2 \mathrm{PO}_3\). - Formed from phosphorous acid (H\(_3\)PO\(_3\)) when not fully neutralized. - This compound has replaceable hydrogen ions, fitting the definition of an acid salt.

Step 4: Examining Option (c)

- Compound: \(\mathrm{Na}_2 \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7\). - Pyrosulfuric acid (H\(_2\)S\(_2\)O\(_7\)) forms this compound when partially neutralized. - Contains replaceable hydrogen ions, making it an acid salt.

Step 5: Examining Option (d)

- Compound: \(\mathrm{NaH}_2 \mathrm{PO}_4\). - Derived from phosphoric acid (H\(_3\)PO\(_4\)) and features replaceable hydrogen ions. - Also fits as an acid salt with replaceable hydrogen.

Step 6: Final Conclusion

Upon reviewing all compounds, \(\mathrm{NaH}_2 \mathrm{PO}_2\) from option (a) does not behave as an acid salt since it does not have additional replaceable hydrogen ions.

Key Concepts

Polybasic Acid

A polybasic acid is a type of acid with more than one hydrogen atom that can be replaced by metal ions or other cations during a reaction. This makes polybasic acids distinct, as they can form various types of salts depending on the degree of neutralization. Examples of polybasic acids include:

• Phosphoric Acid (H₃PO₄): Contains three replaceable hydrogen ions.
• Sulfuric Acid (H₂SO₄): Contains two replaceable hydrogen ions.

When these acids are partially neutralized, they often form acid salts. For instance, when not all hydrogen ions are replaced, the remaining hydrogen can still participate in further reactions. Understanding the behavior of polybasic acids is central to grasping the concept of acid salts, as they provide multiple opportunities for forming new compounds during neutralization.

Replaceable Hydrogen Ion

The concept of replaceable hydrogen ions is central to understanding acids and acid salts. In an acid, a hydrogen ion (H⁺) can be released in solution, often replacing by metal ions in the formation of a salt. Consider these key points:

• Replaceable hydrogen ions determine the strength and properties of an acid.
• The more replaceable hydrogen ions an acid has, the more complex the potential salts it can form.

For example, in phosphoric acid (H₃PO₄), there are three replaceable hydrogen ions. This allows it to form various types of salts, including mono, di, and triprotic salts based on how many hydrogen ions are replaced. Identifying which hydrogen ions can be replaced helps in predicting the possible compounds resulting from partial or complete neutralization.

Partial Neutralization

Partial neutralization occurs when not all the replaceable hydrogen ions in an acid are replaced by metal ions, leading to the formation of an acid salt. This is a crucial step in the chemical reaction between acids and bases, allowing for the development of a variety of salts from a single polybasic acid. Key considerations in partial neutralization:

• The process stops before all hydrogen ions are replaced.
• It results in the formation of an intermediate salt – the acid salt – which retains some acidic property due to remaining hydrogen ions.

In practice, let's say phosphorous acid (H₃PO₃) goes through partial neutralization. It can yield the acid salt NaH₂PO₃ when two hydrogen ions are replaced. This leaves one hydrogen ion intact, enabling the compound to display characteristics of both an acid and a salt. Understanding this process helps clarify how various acid-base reactions yield specific, functional compounds.