How to solve this question using equivalence concept (not using mole concept)

To tackle a problem using the equivalence concept rather than the mole concept, it's essential to first grasp what equivalence means in a chemical context. Equivalence refers to the amount of a substance that reacts with or is equivalent to another substance in a chemical reaction. This approach is particularly useful in titrations and redox reactions, where we often deal with the stoichiometry of reactions without explicitly calculating moles.

Understanding Equivalence in Reactions

In chemistry, equivalence can be defined in terms of reactive capacity. For example, in acid-base reactions, one equivalent of an acid can neutralize one equivalent of a base. The key is to identify the reactive species and their stoichiometric relationships.

Steps to Solve Using the Equivalence Concept

• Identify the Reaction: Determine the chemical reaction you are dealing with. For instance, if you are neutralizing hydrochloric acid (HCl) with sodium hydroxide (NaOH), the reaction can be represented as:

HCl + NaOH → NaCl + H₂O

• Determine the Equivalents: Calculate the number of equivalents for each reactant. For acids and bases, the equivalence is often based on the number of protons (H⁺) or hydroxide ions (OH⁻) they can donate or accept. For example, HCl can donate one proton, so 1 mole of HCl is equivalent to 1 equivalent.
• Set Up the Equivalence Equation: Use the equivalence concept to set up an equation based on the stoichiometry of the reaction. For the reaction above, if you have 0.1 M HCl and you want to neutralize it with NaOH, you can express the equivalence as:

Equivalents of HCl = Equivalents of NaOH

• Calculate the Required Amount: If you know the volume of HCl used, you can calculate the equivalents of HCl. For example, if you use 50 mL of 0.1 M HCl:

Equivalents of HCl = Molarity × Volume = 0.1 mol/L × 0.050 L = 0.005 equivalents

• Find the Volume of NaOH Needed: Since the reaction is 1:1, you will need the same number of equivalents of NaOH. If you have a NaOH solution of 0.1 M, you can find the volume required:

Volume of NaOH = Equivalents of NaOH / Molarity = 0.005 equivalents / 0.1 mol/L = 0.050 L or 50 mL

Example Scenario

Consider a scenario where you are titrating a solution of sulfuric acid (H₂SO₄) with sodium hydroxide (NaOH). Sulfuric acid is a diprotic acid, meaning it can donate two protons. Therefore, 1 mole of H₂SO₄ is equivalent to 2 equivalents. If you have 0.1 M H₂SO₄ and you use 25 mL of it:

Equivalents of H₂SO₄ = 0.1 mol/L × 0.025 L = 0.0025 equivalents (but since it's diprotic, you have 0.005 equivalents).

To neutralize this, you would need 0.005 equivalents of NaOH. If your NaOH solution is also 0.1 M:

Volume of NaOH = 0.005 equivalents / 0.1 mol/L = 0.050 L or 50 mL.

Practical Applications

This equivalence concept is particularly useful in laboratory settings, such as titrations, where precise measurements are critical. By focusing on equivalents, you can simplify calculations and ensure that you are accurately determining the concentrations of reactants and products in a reaction.

In summary, using the equivalence concept allows you to approach chemical reactions from a practical standpoint, focusing on the reactive capacities of substances rather than getting bogged down in mole calculations. This method is especially effective in scenarios involving acid-base neutralizations and redox reactions, where understanding the relationship between reactants is key to solving problems efficiently.