The Yield of a Reaction

Mole Calculations

What is theoretical yield?

The maximum possible mass of product that can be formed, assuming the reaction goes to completion and there are no losses.

What is actual yield?

The actual mass of product obtained experimentally.

What is percentage yield?

The actual yield divided by the theoretical yield, expressed as a percentage.

Why is the actual yield often lower than the theoretical yield?

The reaction may be reversible or incomplete, side reactions may produce unwanted products, or some product may be lost during separation and purification.

How can a reversible reaction reduce the yield?

The reaction may not go to completion, so not all reactants are converted into the desired product.

How can side reactions reduce the yield?

Some reactants form unwanted products instead of the desired product.

How can purification reduce the yield?

Some of the desired product may be lost during processes such as filtration, transfer or purification.

How is theoretical yield calculated?

Use the balanced equation and reacting-mass calculations to calculate the maximum possible amount of product.

What assumption is made when calculating theoretical yield?

The reaction goes to completion with no loss of product.

What are the main steps for calculating theoretical yield?

1. Convert the starting mass to moles. 2. Use the mole ratio from the balanced equation. 3. Convert the moles of product into mass.

What equation is used to calculate percentage yield?

Percentage yield = (actual yield ÷ theoretical yield) × 100.

How do you calculate percentage yield from actual and theoretical yields?

Divide the actual yield by the theoretical yield and multiply by 100.

What units does percentage yield have?

%.

What is the maximum theoretical percentage yield?

100%.

For CuCO₃ → CuO + CO₂, what is the mole ratio CuCO₃ : CuO?

1 : 1.

What is the molar mass of CuCO₃?

123.5 g mol⁻¹.

What is the molar mass of CuO?

79.5 g mol⁻¹.

How many moles are in 5.78 g of CuCO₃?

n = 5.78 ÷ 123.5 = 0.0468 mol.

How many moles of CuO can theoretically form from 0.0468 mol of CuCO₃?

0.0468 mol because the mole ratio is 1 : 1.

What is the theoretical yield of CuO from 5.78 g of CuCO₃?

m = 0.0468 × 79.5 = 3.72 g.

What is the equation for magnesium reacting with phosphoric acid?

3Mg + 2H₃PO₄ → Mg₃(PO₄)₂ + 3H₂.

What is the mole ratio Mg : Mg₃(PO₄)₂?

3 : 1.

How many moles are in 5.62 g of Mg?

n = 5.62 ÷ 24.3 = 0.231 mol.

How many moles of Mg₃(PO₄)₂ can theoretically form from 0.231 mol Mg?

0.231 ÷ 3 = 0.0770 mol.

What is the molar mass of Mg₃(PO₄)₂?

262.9 g mol⁻¹.

What is the theoretical yield of Mg₃(PO₄)₂ from 5.62 g Mg?

m = 0.0770 × 262.9 = 20.2 g.

A reaction has a theoretical yield of 26.7 tonnes and an actual yield of 18.5 tonnes. What is its percentage yield?

(18.5 ÷ 26.7) × 100 = 69.3%.

What is the equation for producing methanol from carbon monoxide and hydrogen?

CO + 2H₂ → CH₃OH.

What is the mole ratio CO : CH₃OH?

1 : 1.

What is the molar mass of CO?

28.0 g mol⁻¹.

What is the molar mass of CH₃OH?

32.0 g mol⁻¹.

How many grams are in 4.32 tonnes?

4.32 × 10⁶ g.

How many moles are in 4.32 tonnes of CO?

(4.32 × 10⁶) ÷ 28.0 = 1.54 × 10⁵ mol.

How many moles of CH₃OH can theoretically form from 1.54 × 10⁵ mol CO?

1.54 × 10⁵ mol because the ratio is 1 : 1.

What is the theoretical yield of CH₃OH from 4.32 tonnes of CO?

4.94 × 10⁶ g = 4.94 tonnes.

If the actual yield of CH₃OH is 4.07 tonnes and the theoretical yield is 4.94 tonnes, what is the percentage yield?

(4.07 ÷ 4.94) × 100 = 82.4%.

How many grams are in 1 kg?

1 × 10³ g.

How many grams are in 1 tonne?

1 × 10⁶ g.

What should you do if an exam question asks why the percentage yield is low?

Give a specific reason linked to the reaction or experimental method.

Give an example of a specific experimental reason for a low yield.

Some product may be left on the filter paper during filtration.

Why is saying simply "some product was lost" often not enough in an exam?

You should explain specifically where or how the product was lost.

What is the difference between theoretical yield and actual yield?

Theoretical yield is the calculated maximum possible amount, whereas actual yield is the amount actually obtained experimentally.

Why is percentage yield useful in industry?

It shows how effectively reactants are converted into the desired product and helps assess the efficiency and economics of a process.