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Comparing the mathematical 'perfect' outcome of a reaction to the actual results obtained in a lab.
Imagine following a recipe for 24 cookies perfectly, yet only 20 come out of the oven. In the world of chemistry, this 'missing' portion isn't a mystery—it's the gap between mathematical perfection and the messy reality of the lab.
In a perfect world, every atom of your limiting reactant would convert into product. This maximum possible amount is called the theoretical yield. To find it, we use stoichiometry to 'bridge' the gap between the mass of our starting material and the mass of our product. We assume the reaction goes to completion and no material is lost. The calculation follows a strict path: grams of reactant moles of reactant mole ratio moles of product grams of product.
If you start with of reacting with excess to produce , what is the theoretical yield?
1. Convert mass to moles: 2. Use the mole ratio (): 3. Convert back to mass:
Theoretical Yield = .
Quick Check
If a reaction is calculated to produce 50 grams of product but only produces 45 grams in the lab, which value is the theoretical yield?
Answer
The 50 grams is the theoretical yield because it is the calculated maximum.
In the lab, you weigh your final product on a balance; this is the actual yield. It is almost always lower than the theoretical yield. To measure the efficiency of your reaction, we calculate the percent yield. This value tells us what percentage of our 'dream' yield we actually achieved. The formula is:
A student calculates that a reaction should produce of copper (theoretical yield). After performing the experiment, they collect and dry the copper, finding it weighs (actual yield). Calculate the percent yield.
1. Identify values: , . 2. Apply formula: .
The reaction was efficient.
Quick Check
Can a percent yield ever be over 100%? If so, does it mean you created matter?
Answer
It can appear over 100% if the product is impure or wet, but it never means matter was created.
Why don't we get ? Advanced chemists must distinguish between mechanical loss and chemical loss. Mechanical loss includes product sticking to filter paper or being left in a beaker. Chemical loss is more complex: it involves side reactions (where reactants form something you didn't want) or equilibrium (where the reaction stops before all reactants are used). If your yield is over , your product is likely contaminated with solvent (it's 'wet') or unreacted starting materials.
You react of with of to form . You collect of . Find the percent yield.
1. Find Limiting Reactant: - Moles : - Moles : - is limiting ( ratio). 2. Calculate Theoretical Yield: . 3. Calculate Percent Yield: .
Which of these is the correct formula for Percent Yield?
If your actual yield is and your theoretical yield is , what is your percent yield?
A percent yield of 105% usually indicates that the experiment was exceptionally efficient and exceeded the laws of stoichiometry.
Review Tomorrow
Tomorrow morning, try to write down the Percent Yield formula from memory and explain to yourself why the 'Actual Yield' is almost always the smaller number.
Practice Activity
Look up a recipe for bread. If the recipe says it makes 2 loaves but you only make 1 large loaf, calculate your 'Loaf Yield' percentage!