Equivalent Mass Of Oxalic Acid: Calculation & Explanation

by Wholesomestory Johnson 58 views

Hello! Today, we'll tackle the question of finding the equivalent mass of oxalic acid. We'll break down the concept, calculate the equivalent mass, and explain the underlying chemistry. Let's get started!

Correct Answer

The equivalent mass of oxalic acid (H₂C₂O₄) depends on the reaction it undergoes. For reactions where both acidic hydrogens are replaced, the equivalent mass is 63.01 g/equivalent. For reactions where only one acidic hydrogen is replaced, the equivalent mass is 126.03 g/equivalent.

Detailed Explanation

To understand the equivalent mass of oxalic acid, we first need to grasp a few key concepts. Then, we'll delve into the calculation and different scenarios.

Key Concepts

  • Equivalent Mass: The equivalent mass of a substance is the mass of the substance that will combine with or displace a fixed amount of another substance. For acids, it's the molar mass divided by the number of replaceable hydrogen ions (protons).
  • Oxalic Acid (H₂C₂O₄): Oxalic acid is a dicarboxylic acid, meaning it has two carboxyl (-COOH) groups. This means it can donate two protons (H⁺ ions) in a chemical reaction.
  • Molar Mass: The molar mass of a compound is the mass of one mole of that compound. It's calculated by summing the atomic masses of all the atoms in the molecule.
  • Basicity: The basicity of an acid refers to the number of replaceable hydrogen ions (H⁺) present in one molecule of the acid. Oxalic acid is a dibasic acid because it has two replaceable hydrogen ions.

Calculating the Equivalent Mass of Oxalic Acid

The formula to calculate the equivalent mass of an acid is:

Equivalent Mass = Molar Mass / Basicity

Let's break down the calculation step-by-step:

  1. Determine the Molar Mass of Oxalic Acid (H₂C₂O₄):

    • Hydrogen (H): 1.008 g/mol (x2 = 2.016 g/mol)
    • Carbon (C): 12.01 g/mol (x2 = 24.02 g/mol)
    • Oxygen (O): 16.00 g/mol (x4 = 64.00 g/mol)

    Molar Mass (H₂C₂O₄) = 2.016 + 24.02 + 64.00 = 90.036 g/mol

    However, oxalic acid commonly exists as a dihydrate (H₂C₂O₄ · 2H₂O). So, let's calculate the molar mass of oxalic acid dihydrate as well:

    • Water (H₂O): (1.008 g/mol x 2) + 16.00 g/mol = 18.016 g/mol (x2 = 36.032 g/mol)

    Molar Mass (H₂C₂O₄ · 2H₂O) = 90.036 + 36.032 = 126.068 g/mol

  2. Determine the Basicity of Oxalic Acid:

    Oxalic acid (H₂C₂O₄) has two replaceable hydrogen ions (H⁺), so its basicity is 2.

  3. Calculate the Equivalent Mass:

    Now, we apply the formula:

    Equivalent Mass = Molar Mass / Basicity

    • For anhydrous oxalic acid (H₂C₂O₄):

      Equivalent Mass = 90.036 g/mol / 2 = 45.018 g/equivalent

    • For oxalic acid dihydrate (H₂C₂O₄ · 2H₂O):

      Equivalent Mass = 126.068 g/mol / 2 = 63.034 g/equivalent

Scenarios and Different Equivalent Masses

It's crucial to note that the equivalent mass can vary depending on the chemical reaction. Oxalic acid, being a dibasic acid, can react in two main ways:

  1. Reaction where both acidic hydrogens are replaced: In this case, the basicity is 2, and we use the calculations we just did.
    • For the dihydrate form, this gives us an equivalent mass of approximately 63.01 g/equivalent (rounding 63.034).
  2. Reaction where only one acidic hydrogen is replaced: In certain reactions, only one of the two protons (H⁺) from oxalic acid is involved. This effectively makes the basicity 1.
    • For the dihydrate form, the equivalent mass would then be: Equivalent Mass = 126.068 g/mol / 1 = 126.03 g/equivalent (rounding 126.068).

Examples to Illustrate the Concept

Let's look at some examples to clarify when each equivalent mass is applicable:

  1. Titration with a strong base (NaOH) where both protons react:

    H₂C₂O₄ + 2 NaOH → Na₂C₂O₄ + 2 H₂O

    In this reaction, both protons from oxalic acid react with the sodium hydroxide. Therefore, the basicity is 2, and the equivalent mass of oxalic acid dihydrate is 63.01 g/equivalent.

  2. Reaction with KMnO₄ in acidic medium:

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

    In this redox reaction, oxalic acid acts as a reducing agent and loses both of its protons during oxidation. Thus, the basicity is considered as 2, and the equivalent mass is 63.01 g/equivalent.

  3. Formation of an acid salt where only one proton is replaced:

    H₂C₂O₄ + NaOH → NaHC₂O₄ + H₂O

    Here, only one proton from oxalic acid reacts, forming sodium hydrogen oxalate (an acid salt). In this case, the basicity is 1, and the equivalent mass of oxalic acid dihydrate is 126.03 g/equivalent.

Why is Equivalent Mass Important?

Understanding equivalent mass is crucial in several areas of chemistry, particularly in:

  • Volumetric Analysis (Titration): Equivalent mass is used to calculate the normality of solutions, which is vital for titrations. Normality represents the number of equivalents of solute per liter of solution.
  • Neutralization Reactions: Knowing the equivalent masses of acids and bases helps determine the amounts needed to completely neutralize each other.
  • Stoichiometry: Equivalent mass simplifies stoichiometric calculations, especially in redox reactions and reactions involving polyprotic acids like oxalic acid.

Common Mistakes to Avoid

  • Forgetting to consider the dihydrate form: Always check if the oxalic acid is anhydrous or dihydrate, as this affects the molar mass and, consequently, the equivalent mass.
  • Using the wrong basicity: Identify the reaction type to determine whether one or both protons are involved. This will dictate whether the basicity is 1 or 2.
  • Confusing equivalent mass with molar mass: Remember that equivalent mass is molar mass divided by basicity (or valency factor in other contexts).

Key Takeaways

  • The equivalent mass of oxalic acid depends on whether it's anhydrous (H₂C₂O₄) or dihydrate (H₂C₂O₄ · 2H₂O) and the reaction it undergoes.
  • For reactions where both protons are replaced, the equivalent mass of oxalic acid dihydrate is approximately 63.01 g/equivalent.
  • For reactions where only one proton is replaced, the equivalent mass of oxalic acid dihydrate is approximately 126.03 g/equivalent.
  • Always calculate the molar mass correctly, considering the hydrated form if applicable.
  • Determine the basicity (number of replaceable protons) based on the specific reaction.
  • Equivalent mass is essential for calculations in titrations, neutralization reactions, and stoichiometry.

I hope this explanation clarifies the concept of the equivalent mass of oxalic acid and its calculation. If you have any more questions, feel free to ask!