Complete Guide to Oxalic Acid Formulas

Complete Table of Oxalic Acid Formulas

Detailed Explanations of Key Formulas

1. Anhydrous Oxalic Acid Formula: C₂H₂O₄

• Description: Pure oxalic acid without any water molecules
• Molecular Weight: 90.03 g/mol
• Appearance: White crystalline solid
• Usage: Less common in laboratories; primarily used in industry

2. Oxalic Acid Dihydrate Formula: C₂H₂O₄·2H₂O

• Description: Crystalline form containing two water molecules per oxalic acid molecule
• Molecular Weight: 126.07 g/mol (90.03 + 36.04 from 2H₂O)
• Appearance: Colorless or white transparent crystals
• Usage: Most commonly available form in laboratories and commercial applications
• Water Content: Contains approximately 28.6% water by mass

3. Structural Formula: HOOC-COOH

• Description: Shows the actual arrangement of atoms
• Key Features:
  • Two carboxyl groups (-COOH) directly bonded
  • Each carboxyl group can donate one H⁺ ion (diprotic acid)
  • Bond angle and structure determine its reactivity

Important Chemical Properties Related to Formulas

Conversion Between Different Forms

Important Calculation:

• 1 mole of C₂H₂O₄·2H₂O (126.07 g) = 1 mole of C₂H₂O₄ (90.03 g) + 2 moles of H₂O (36.04 g)
• To convert dihydrate to anhydrous: Multiply by (90.03/126.07) = 0.714
• To convert anhydrous to dihydrate: Multiply by (126.07/90.03) = 1.400

Frequently Asked Questions about Oxalic Acid Formulas

Q. What is the chemical formula of oxalic acid?

The chemical formula of oxalic acid in its anhydrous form is C₂H₂O₄. However, the most commonly encountered form in laboratories is oxalic acid dihydrate with the formula C₂H₂O₄·2H₂O.

Q. What is the difference between oxalic acid and oxalic acid dihydrate?

• Oxalic acid (C₂H₂O₄): Anhydrous form without water molecules, molecular weight 90.03 g/mol
• Oxalic acid dihydrate (C₂H₂O₄·2H₂O): Contains two water molecules per acid molecule, molecular weight 126.07 g/mol
• The dihydrate is more stable and commonly used in practice.

Q. Why is oxalic acid called a dicarboxylic acid?

Oxalic acid is called a dicarboxylic acid because it contains two carboxylic acid groups (-COOH) in its structure, as shown by the formula HOOC-COOH. Each carboxyl group can donate one proton (H⁺).

Q. What is the molecular weight of oxalic acid dihydrate?

The molecular weight of oxalic acid dihydrate (C₂H₂O₄·2H₂O) is 126.07 g/mol, calculated as:

• C₂H₂O₄: 90.03 g/mol
• 2H₂O: 36.04 g/mol
• Total: 126.07 g/mol

Q. How do you write the structural formula of oxalic acid?

The structural formula of oxalic acid is:

O=C(OH)-C(OH)=O or HOOC-COOH

This shows two carboxylic acid groups bonded directly to each other.

Q. What is the empirical formula of oxalic acid?

The empirical formula of oxalic acid is CHO₂, which represents the simplest whole-number ratio of atoms. This is derived by dividing the molecular formula C₂H₂O₄ by 2.

Q. Is oxalic acid monobasic or dibasic?

Oxalic acid is dibasic (diprotic) because it has two ionizable hydrogen atoms and can donate two protons in solution:

• First dissociation: H₂C₂O₄ → H⁺ + HC₂O₄⁻
• Second dissociation: HC₂O₄⁻ → H⁺ + C₂O₄²⁻

Q. What happens when oxalic acid dihydrate is heated?

When heated, oxalic acid dihydrate first loses its water of crystallization at around 101-102°C, converting to anhydrous oxalic acid (C₂H₂O₄). Upon further heating to 189.5°C, it decomposes into formic acid, carbon dioxide, carbon monoxide, and water.

Q. How is the oxalate ion formula different from oxalic acid?

• Oxalic acid: H₂C₂O₄ (neutral molecule with both H atoms)
• Hydrogen oxalate: HC₂O₄⁻ (lost one H⁺, single negative charge)
• Oxalate ion: C₂O₄²⁻ (lost both H⁺ ions, double negative charge)

The oxalate ion is the conjugate base formed when oxalic acid completely dissociates.

Q. Why do we use oxalic acid dihydrate instead of anhydrous form in titrations?

Oxalic acid dihydrate is preferred because:

• It is more stable and less hygroscopic (doesn’t absorb moisture from air easily)
• It is easier to weigh accurately as a primary standard
• It has a well-defined composition with consistent water content
• The anhydrous form is more difficult to handle and store properly

Academic Applications

For School Students (Grades 9-12):

• Understanding molecular and structural formulas
• Calculating molecular weights
• Stoichiometry problems involving hydrated compounds

For College Students:

• Acid-base equilibrium calculations using pKa values
• Complexometric titrations with oxalate ions
• Organic chemistry reactions of dicarboxylic acids
• Analytical chemistry applications as a primary standard

Important Notes:

• The basic molecular formula is C₂H₂O₄ (anhydrous form)
• The common laboratory form is C₂H₂O₄·2H₂O (dihydrate)
• Oxalic acid is a diprotic/dibasic acid with two -COOH groups
• Always check which form is being used when performing calculations
• Understanding the difference between hydrated and anhydrous forms is crucial for accurate quantitative analysis

Note for Students: When performing calculations in laboratory work or exams, always verify whether the question refers to anhydrous oxalic acid (90.03 g/mol) or oxalic acid dihydrate (126.07 g/mol), as using the wrong molecular weight will result in incorrect answers.