Equivalent Weight Of Oxalic Acid: A Complete Guide

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Hello there! I understand you're curious about the equivalent weight of oxalic acid. Don't worry, I'm here to give you a clear, detailed, and correct explanation. Let's dive in!

Correct Answer

The equivalent weight of oxalic acid (H₂C₂O₄) is 63 g/mol.

Detailed Explanation

Oxalic acid is a dicarboxylic acid, meaning it has two carboxyl groups (-COOH). It's a common chemical compound used in various applications, from cleaning to removing rust and even in some laboratory experiments. Understanding its equivalent weight is crucial for performing accurate chemical calculations, especially in titrations and stoichiometry problems. Let's break down the concept of equivalent weight and how to calculate it for oxalic acid.

Key Concepts

Before we jump into the calculation, let's define some key terms:

  • Molecular Weight: This is the sum of the atomic weights of all the atoms in a molecule. It's expressed in grams per mole (g/mol). You can find this by adding up the atomic masses from the periodic table.
  • Equivalent Weight: This is the mass of a substance that will react with or replace a fixed amount of another substance. For acids, it's the mass of the acid that provides one mole of hydrogen ions (H⁺). For bases, it's the mass of the base that provides one mole of hydroxide ions (OH⁻). For redox reactions, it's the mass of the substance that gains or loses one mole of electrons.
  • Basicity: This refers to the number of replaceable hydrogen ions (H⁺) an acid can donate per molecule. For oxalic acid (H₂C₂O₄), the basicity is 2 because it can donate two H⁺ ions.
  • Molarity: The number of moles of solute per liter of solution (mol/L).
  • Normality: The number of gram equivalents of solute per liter of solution (N). In simpler terms, normality is the number of equivalents of solute present in one liter of solution.

Calculating the Equivalent Weight

The formula to calculate equivalent weight is:

Equivalent Weight = (Molecular Weight) / (Basicity or n-factor)

Let's apply this formula to oxalic acid (H₂C₂O₄):

  1. Determine the Molecular Weight:
    • The molecular formula for oxalic acid is H₂C₂O₄.
    • The atomic masses are approximately:
      • Hydrogen (H): 1 g/mol
      • Carbon (C): 12 g/mol
      • Oxygen (O): 16 g/mol
    • So, the molecular weight calculation is:
      • (2 x 1) + (2 x 12) + (4 x 16) = 2 + 24 + 64 = 90 g/mol
  2. Determine the Basicity:
    • Oxalic acid (H₂C₂O₄) has two replaceable hydrogen atoms. Therefore, its basicity is 2.
  3. Calculate the Equivalent Weight:
    • Equivalent Weight = (Molecular Weight) / (Basicity)
    • Equivalent Weight = 90 g/mol / 2
    • Equivalent Weight = 45 g/equivalent

However, it is important to consider whether the oxalic acid is hydrated or anhydrous. The most common form is dihydrated oxalic acid (H₂C₂O₄ · 2H₂O). If the question refers to dihydrated oxalic acid, the calculation changes slightly:

  1. Determine the Molecular Weight of Dihydrated Oxalic Acid:
    • Molecular weight of H₂C₂O₄ is 90 g/mol
    • Molecular weight of 2H₂O is 2 * (18) = 36 g/mol
    • Total Molecular Weight = 90 + 36 = 126 g/mol
  2. Determine the Basicity:
    • As before, the basicity is 2.
  3. Calculate the Equivalent Weight:
    • Equivalent Weight = (Molecular Weight) / (Basicity)
    • Equivalent Weight = 126 g/mol / 2
    • Equivalent Weight = 63 g/equivalent

*Important Note: In many chemical reactions, especially titrations, the dihydrated form of oxalic acid (H₂C₂O₄ · 2H₂O) is used. Therefore, it's crucial to know which form is being referred to in your calculations.

Example Problem

Let's look at a sample problem to understand this better:

Problem: What is the molarity of a solution containing 6.3 g of oxalic acid dihydrate (H₂C₂O₄ · 2H₂O) in 500 mL of solution?

Solution:

  1. Calculate the number of moles of oxalic acid dihydrate:
    • Molecular weight of oxalic acid dihydrate = 126 g/mol
    • Moles = mass / molecular weight = 6.3 g / 126 g/mol = 0.05 mol
  2. Convert volume to liters:
    • 500 mL = 0.5 L
  3. Calculate the molarity:
    • Molarity = moles / volume (in liters) = 0.05 mol / 0.5 L = 0.1 M

Therefore, the molarity of the solution is 0.1 M.

Oxalic Acid in Redox Reactions

Oxalic acid can also participate in redox reactions. In these reactions, the equivalent weight depends on the number of electrons transferred.

For example, when oxalic acid is oxidized to carbon dioxide (CO₂) in the presence of an oxidizing agent (like potassium permanganate, KMnO₄), the equivalent weight calculation changes.

The balanced equation for this redox reaction (in acidic medium) is:

5 H₂C₂O₄ + 2 KMnO₄ + 3 H₂SO₄ → 2 MnSO₄ + 10 CO₂ + K₂SO₄ + 8 H₂O

In this reaction, each molecule of oxalic acid loses two electrons.

So, the equivalent weight of oxalic acid in this redox reaction is:

Equivalent Weight = (Molecular Weight) / (Number of electrons transferred)

For anhydrous oxalic acid: Equivalent Weight = 90 g/mol / 2 = 45 g/equivalent

For oxalic acid dihydrate: Equivalent Weight = 126 g/mol / 2 = 63 g/equivalent

Titration with KMnO₄

Titration is a common laboratory technique used to determine the concentration of a substance. When titrating oxalic acid with potassium permanganate (KMnO₄), you're performing a redox titration.

Here's how it works:

  1. Prepare the Oxalic Acid Solution:
    • Weigh a known mass of oxalic acid (or oxalic acid dihydrate) and dissolve it in a known volume of water.
  2. Prepare the KMnO₄ Solution:
    • KMnO₄ solution is usually prepared and standardized (its exact concentration is determined). KMnO₄ acts as the oxidizing agent.
  3. The Titration Process:
    • Oxalic acid solution is placed in a flask, and a few drops of sulfuric acid (H₂SO₄) are added to provide the acidic medium for the reaction.
    • The KMnO₄ solution is added from a burette (a graduated glass tube with a tap) drop by drop.
    • As KMnO₄ is added, it reacts with the oxalic acid. The MnO₄⁻ ion is reduced to Mn²⁺ ion. The solution turns from colorless to a faint pink color when all the oxalic acid has reacted, and the first excess drop of KMnO₄ remains in the solution. This is the endpoint of the titration.
  4. Calculations:
    • Using the balanced equation and the volume of KMnO₄ used, you can calculate the concentration of oxalic acid or the concentration of KMnO₄.

Real-World Applications

Oxalic acid and its salts have a variety of uses:

  • Cleaning: Oxalic acid is a good cleaning agent, often used to remove rust stains and other types of stains from surfaces. It works by forming a soluble complex with the metal ions in the rust, which can then be washed away.
  • Bleaching: It is also used as a bleaching agent in the textile industry.
  • Metal Polishing: It is used to polish metals.
  • Analysis: It's a primary standard in analytical chemistry, used to standardize solutions of bases (like sodium hydroxide, NaOH) and oxidizing agents (like potassium permanganate, KMnO₄).

Key Takeaways

  • The equivalent weight of anhydrous oxalic acid (H₂C₂O₄) is 45 g/equivalent.
  • The equivalent weight of oxalic acid dihydrate (H₂C₂O₄ · 2H₂O) is 63 g/equivalent.
  • Basicity (for acid-base reactions) is 2 for both forms.
  • In redox reactions, the equivalent weight also depends on the number of electrons transferred.
  • Oxalic acid is used in cleaning, bleaching, and as a primary standard in analytical chemistry.

I hope this comprehensive guide has helped you understand the equivalent weight of oxalic acid! If you have any more questions, feel free to ask!