Percentage Purity & Yield
Stoichiometry | Some Basic Concepts of Chemistry Class 11
1. Percentage Purity
In real-world chemistry, reactants used in the laboratory or industry are rarely 100% pure. They often contain impurities (sand, moisture, other salts). Stoichiometric calculations must be performed using only the mass of the pure reactant.
Note: Impurities do not participate in the specific chemical reaction being studied.
$$ \% \text{ Purity} = \frac{\text{Mass of Pure Substance}}{\text{Mass of Impure Sample}} \times 100 $$
Steps for Calculation:
- If Product mass is given: Use stoichiometry to find the mass of pure reactant required.
- Calculate Percentage Purity using the given sample mass.
- If Purity is given: Mass of pure reactant = $\frac{\text{Purity}}{100} \times \text{Sample Mass}$.
2. Percentage Yield
Ideally, 100% of the limiting reagent converts to products. However, in practice, the amount of product obtained is often less than calculated. This efficiency is measured by Percentage Yield.
- Theoretical Yield: The maximum amount of product that can be produced from a given amount of reactant, calculated using stoichiometry.
- Actual Yield: The amount of product actually obtained from the experiment.
$$ \% \text{ Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100 $$
Why is Actual Yield less than Theoretical Yield?
- Side Reactions: Reactants may form other by-products.
- Incomplete Reaction: The reaction may be reversible or reach equilibrium.
- Mechanical Loss: Loss during filtration, transfer, or crystallization.
- Impure Reactants: As discussed in the purity section.
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