Percent Yield: Videos & Practice Problems

Stoichiometry allows us to calculate the theoretical yield of a chemical reaction, while percent yield measures the actual success of the experiment. The formula for percent yield is actual yieldtheoretical yield⁢100. Percent yields can be categorized as excellent (≥90%), very good (≥80%), good (≥70%), or poor (<40%). Actual yield is typically less than theoretical yield due to inefficiencies in reactions, such as loss of material or external factors affecting the process.

Percent Yield

Stoichiometry is a crucial concept in chemistry that allows us to calculate the theoretical amount of product that can be produced from a given quantity of reactants. However, the real measure of success in a chemical reaction is determined by the percent yield, which reflects the efficiency of the reaction. A higher percent yield indicates a more successful synthesis of the desired product, similar to aiming for high percentages on an exam.

Percent yield can be categorized into different levels of success: an excellent yield is defined as 90% or greater, a very good yield ranges from 80% to 89%, a good yield is between 70% and 79%, and a poor yield is anything below 40%. Understanding these classifications helps in evaluating the effectiveness of a chemical reaction.

The formula for calculating percent yield is given by:

\[\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\]

This formula is essential to memorize, as it is often not provided on formula sheets. In this context, the actual yield refers to the amount of pure product obtained from the experiment, while the theoretical yield is the amount calculated based on stoichiometric principles.

It is important to note that the units of actual yield and theoretical yield must match when using the formula. For instance, if the actual yield is measured in grams, the theoretical yield must also be converted to grams from moles if necessary. Additionally, it is a fundamental principle that no chemical reaction can achieve a 100% yield due to various factors such as loss of material during handling or external influences. Therefore, the actual yield is always less than the theoretical yield.

In summary, percent yield, actual yield, and theoretical yield are interconnected concepts that provide valuable insights into the efficiency of chemical reactions, helping chemists assess their experimental success.

Percent Yield Example 1

In a balanced chemical reaction involving benzene (C₆H₆) and oxygen, the stoichiometry plays a crucial role in determining the percent yield of a product, in this case, water (H₂O). The reaction can be summarized as:

2 C₆H₆ + 15 O₂ → 12 CO₂ + 6 H₂O

To calculate the percent yield of water produced from a 2.6 gram sample of benzene that yields 1.25 grams of water, we first need to establish the theoretical yield using stoichiometry. The percent yield formula is:

Percent Yield = (Actual Yield / Theoretical Yield) × 100

Here, the actual yield of water is given as 1.25 grams. To find the theoretical yield, we start by converting the mass of benzene to moles. The molar mass of benzene is calculated as follows:

Molar Mass of C₆H₆ = (6 × 12.011 g/mol) + (6 × 1.008 g/mol) = 78.108 g/mol

Next, we convert the grams of benzene to moles:

Moles of C₆H₆ = 2.6 g / 78.108 g/mol ≈ 0.0333 moles

Using the stoichiometric coefficients from the balanced equation, we find the moles of water produced. The ratio of moles of C₆H₆ to moles of H₂O is 2:6, or 1:3. Therefore:

Moles of H₂O = 0.0333 moles C₆H₆ × (6 moles H₂O / 2 moles C₆H₆) = 0.0999 moles H₂O

Now, we convert moles of water to grams using the molar mass of water, which is:

Molar Mass of H₂O = (2 × 1.008 g/mol) + (16.00 g/mol) = 18.016 g/mol

Theoretical Yield of H₂O = 0.0999 moles × 18.016 g/mol ≈ 1.799 grams

Now that we have both the actual yield (1.25 grams) and the theoretical yield (1.799 grams), we can calculate the percent yield:

Percent Yield = (1.25 g / 1.799 g) × 100 ≈ 69.48%

This calculation illustrates the application of stoichiometry in determining the efficiency of a chemical reaction, highlighting the importance of understanding both actual and theoretical yields in chemical processes.

What is the percent yield for a reaction in which 22.1 g Cu is isolated by reacting 45.5 g Zn with 70.1 g CuSO₄?  
Zn (s) + CuSO₄ (aq) → Cu (s) + ZnSO₄ (aq)

• A. 63.1 %
• B. 90.7 %
• C. 79.2 %
• D. 67.1 %
• E. 50.0 %

Answer: (C)

The reduction of iron (III) oxide creates the following reaction: 
Fe₂O₃ (s) + 3 H₂ (g) → 2 Fe (s) + 3 H₂O (g) 
If the above reaction only went to 75% completion, how many moles of Fe₂O₃ were require to produce 0.850 moles of Fe? 

• A. 0.963 mol
• B. 2.27 mol
• C. 0.425 mol
• D. 0.567 mol
• E. 1.70 mol

Answer: (D)

Here’s what students ask on this topic:

What is the formula for calculating percent yield in chemistry?

The formula for calculating percent yield in chemistry is:

$\frac{\mathrm{Actual\; Yield}}{\mathrm{Theoretical\; Yield}}100$

Here, the actual yield is the amount of product obtained from the experiment, while the theoretical yield is the amount predicted by stoichiometric calculations. This formula helps determine the efficiency of a chemical reaction.

Why is it impossible to achieve a 100% percent yield in a chemical reaction?

It is impossible to achieve a 100% percent yield in a chemical reaction because no reaction is perfectly efficient. Various factors such as incomplete reactions, side reactions, loss of product during transfer, and measurement inaccuracies contribute to a lower actual yield compared to the theoretical yield. Additionally, external factors like temperature and pressure variations can also affect the yield. Therefore, the actual yield is always less than the theoretical yield, making a 100% percent yield unattainable.

How do you categorize percent yields in terms of efficiency?

Percent yields can be categorized based on their efficiency as follows:

• Excellent: ≥90%
• Very Good: ≥80%
• Good: ≥70%
• Poor: <40%

These categories help in assessing the success of a chemical reaction. Higher percent yields indicate more efficient reactions, while lower yields suggest inefficiencies or issues in the experimental process.

What is the difference between actual yield and theoretical yield?

The actual yield is the amount of product obtained from an experiment, while the theoretical yield is the amount predicted by stoichiometric calculations based on the reactants used. The actual yield is usually less than the theoretical yield due to various factors such as incomplete reactions, side reactions, and product loss. The theoretical yield represents the maximum possible amount of product that could be formed under ideal conditions.

How do you convert units when calculating percent yield?

When calculating percent yield, it is essential to ensure that the units of the actual yield and theoretical yield match. If the actual yield is given in grams and the theoretical yield is in moles, you must convert the theoretical yield to grams using the molar mass of the product. This ensures consistency in the units, allowing for an accurate calculation of percent yield.