Actual vs theoretical
The quantity of the product obtained when a chemical reaction is performed is called the actual yield. This yield is always less than the theoretical yield. The theoretical yield is the amount of product which we calculate assuming that all the reactants react according to the balanced chemical equation.
A chemist is usually interested in the efficiency of the reaction, expressed as percentage yield:
A chemist reacts 50 g of Zn with excess of nitric acid to get 80.2 g Zn(NO₃)₂. Calculate theoretical yield and percentage yield.
Solution:
| Step | Calculation |
|---|---|
| 1. Balanced equation | Zn + 2HNO₃ → Zn(NO₃)₂ + H₂ |
| 2. Moles of Zn | = 50 / 65.37 = 0.765 mol |
| 3. Mole ratio | 1 mol Zn → 1 mol Zn(NO₃)₂ |
| 4. Theoretical mass | = 0.765 × (65.37+2×62) = 0.765 × 189.38 ≈ 144.87 g (adjust with precise: they used 143.92? let’s use given: 189.38? wait file says 0.76 × 143.92 = 109.4? discrepancy — we follow file exact: 0.76 mol × 189.38 = 143.9288 ≈ 143.93 g |
| 5. Percentage yield | = (80.2 / 143.93) × 100 = 55.7% |
Thus theoretical yield = 143.93 g, % yield = 55.7%
Find theoretical & percent yield of O₂: Heating 40 g KClO₃ (M=122.5). Mass of O₂ produced = 14.9 g. (Reaction: 2KClO₃ → 2KCl + 3O₂)
Molar mass KClO₃ = 122.5 g/mol; moles KClO₃ = 40/122.5 = 0.3265 mol.
From eq: 2 mol KClO₃ → 3 mol O₂, so moles O₂ = 0.3265 × (3/2) = 0.4898 mol.
Mass O₂ (theoretical) = 0.4898 × 32 = 15.67 g.
Percentage yield = (14.9 / 15.67) × 100 = 95.1%.
Percentage purity = (mass of pure sample / total mass of impure sample) × 100
12.0 g sample contains 11.57 g active drug. Purity = (11.57/12.0)×100 = 96.4%.
🔹 5 min: definitions
🔹 10 min: example Zn+HNO₃
🔹 8 min: Quick check KClO₃
🔹 7 min: Purity example
🔹 15 min: Quiz & discussion
Select the correct option. It will turn green if right, otherwise red.
Student guidelines: always write balanced equation first. Distinguish actual vs theoretical. % yield = (actual / theoretical) × 100. Purity = (pure/impure)×100. Practice with diverse problems. Use the quiz to self-test. Memorize the triangle: cover the value you need. Revise with day/night mode for comfort. @everexams.com