Equivalent weight, a concept primarily used in chemistry, represents the mass of a substance that can combine with or displace a fixed quantity of another substance. While less frequently used now than molar mass, understanding equivalent weight is still crucial for certain chemical calculations, especially in titration and redox reactions. This guide will walk you through how to calculate equivalent weight for various substances.
Understanding Equivalent Weight
Before diving into the calculations, let's solidify the definition. Equivalent weight is defined differently depending on the type of reaction:
- For acids: Equivalent weight is the mass of the acid that contains one mole of replaceable hydrogen ions (H⁺).
- For bases: Equivalent weight is the mass of the base that contains one mole of replaceable hydroxide ions (OH⁻).
- For salts: Equivalent weight is the mass of the salt that contains one mole of the cation or anion.
- For redox reactions: Equivalent weight is the molar mass of the substance divided by the number of electrons gained or lost per mole in the redox reaction.
Calculating Equivalent Weight: Step-by-Step Examples
Let's illustrate with examples across different types of reactions:
1. Calculating Equivalent Weight of an Acid
Example: Calculate the equivalent weight of sulfuric acid (H₂SO₄).
Solution:
- Identify the number of replaceable hydrogen ions: Sulfuric acid has two replaceable hydrogen ions (H⁺).
- Find the molar mass of the acid: The molar mass of H₂SO₄ is approximately 98 g/mol (21 + 32 + 416).
- Calculate the equivalent weight: Equivalent weight = Molar mass / Number of replaceable H⁺ ions = 98 g/mol / 2 = 49 g/eq.
Therefore, the equivalent weight of sulfuric acid is 49 g/eq.
2. Calculating Equivalent Weight of a Base
Example: Calculate the equivalent weight of calcium hydroxide Ca(OH)₂.
Solution:
- Identify the number of replaceable hydroxide ions: Calcium hydroxide has two replaceable hydroxide ions (OH⁻).
- Find the molar mass of the base: The molar mass of Ca(OH)₂ is approximately 74 g/mol (40 + 2*(16+1)).
- Calculate the equivalent weight: Equivalent weight = Molar mass / Number of replaceable OH⁻ ions = 74 g/mol / 2 = 37 g/eq.
Therefore, the equivalent weight of calcium hydroxide is 37 g/eq.
3. Calculating Equivalent Weight in Redox Reactions
Example: Calculate the equivalent weight of potassium permanganate (KMnO₄) in an acidic medium where it's reduced to Mn²⁺.
Solution:
- Write the balanced redox reaction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
- Determine the number of electrons transferred: 5 electrons are transferred per mole of KMnO₄.
- Find the molar mass of KMnO₄: The molar mass of KMnO₄ is approximately 158 g/mol (39 + 55 + 4*16).
- Calculate the equivalent weight: Equivalent weight = Molar mass / Number of electrons transferred = 158 g/mol / 5 = 31.6 g/eq.
Therefore, the equivalent weight of KMnO₄ in this acidic redox reaction is 31.6 g/eq.
Factors Affecting Equivalent Weight
Remember that the equivalent weight of a substance isn't constant; it depends on the specific reaction it's involved in. The number of replaceable ions or electrons transferred directly influences the calculated equivalent weight. Always carefully consider the reaction context when determining equivalent weight.
Conclusion
Calculating equivalent weight is a fundamental skill in chemistry. By understanding the principles outlined above and practicing with different examples, you'll be able to confidently tackle equivalent weight calculations in various chemical contexts. Remember to always clearly identify the reaction type and the number of replaceable ions or electrons involved.
