GCSE · Chemistry · AQA · Spec 8462

Percentage yield

A reaction can be perfectly planned and still give less product than the calculation predicts.

CHEMISTRY · YIELD

Make percentage yield visible
01= 4/5

actual mass ÷ theoretical mass: 16/20. percentage 80%. Actual product mass 16 g

Fraction16/20Percentage80%Actual product mass16 gTry 10 g, 15 g and 20 g. What percentage does each represent?

In this model the theoretical maximum is fixed at 20 g. Drag the actual amount and watch the fraction and percentage move together.

Watch out: The actual amount goes on top: actual ÷ theoretical, not the other way round.

The percentage-yield relationship

percentage yield = (actual mass ÷ theoretical mass) × 100

Divide what you actually obtained by the maximum theoretical amount, then multiply by 100 to express the result as a percentage.

Actual and theoretical amounts must be for the same product and in compatible units.

From masses to a percentage

percentage yield = (actual ÷ theoretical) × 100

Start with actual over theoretical so the comparison cannot accidentally be reversed.

1 / 4

Higher challenge: theoretical mass first

Problem

12 g of magnesium reacts with oxygen: 2Mg + O₂ → 2MgO. The reaction produces 16 g of magnesium oxide. Calculate the percentage yield. Use Ar: Mg = 24, O = 16.

UK note

Higher-tier route: calculating the theoretical product mass from a reactant mass and a balanced equation is Higher-only content.

See where the theoretical ratio comes from

Read the coefficients as mole ratios, not as masses.

Tap a molecule to see what it does in the reaction.

UK note

Higher-tier route: use a balanced equation to calculate the theoretical mass before finding percentage yield.

CHEMISTRY · QUANTITATIVE

Percentage yield

Compare what you actually make with the maximum amount the reaction could theoretically make.

What you need to know

  • Yield is the amount of product actually obtained from a reaction.
  • Percentage yield compares actual product with the maximum theoretical amount: (actual ÷ theoretical) × 100.
  • Percentage yield may be below 100% because a reaction may be reversible, product may be lost during separation, or reactants may react in unexpected ways.
  • Higher-only: a balanced equation can be used with a reactant mass to calculate the theoretical mass of product.

The big picture

Percentage yield compares the amount of product actually obtained with the maximum theoretical amount. It is calculated as (actual ÷ theoretical) × 100. A yield can be below 100% because a reversible reaction may not go to completion, product can be lost during separation, or reactants can form unexpected products. For higher calculations, use the balanced equation to find the theoretical product mass first.

Key points

1Actual yield means what you really obtain; theoretical yield means the maximum predicted amount.
2Put actual yield over theoretical yield, then multiply by 100.
3A percentage yield of 80% means the actual product is 80% of the theoretical maximum.
4Losses, reversibility and unexpected reactions can reduce the amount of desired product collected.
5For a higher theoretical-mass calculation, use the balanced equation and relative formula masses before applying the percentage-yield formula.

Worked example

Problem

A reaction has a theoretical product mass of 50 g and produces 35 g. Calculate the percentage yield.

🧠

Memory hook

A-T-100: Actual over Theoretical, then ×100.

★ Exam tip

Write actual ÷ theoretical before entering numbers. If a theoretical mass must be calculated first, keep that working separate and clearly labelled.

⚠ Watch out

Dividing theoretical by actual. That reverses the comparison and can give a percentage above 100 for an ordinary yield question.

✓

Check yourself

A reaction should make 40 g but produces 30 g. What is its percentage yield, and name one reason it could be below 100%?

Flashcards

(20)
What is the yield of a reaction?
The amount of product actually obtained.
What is theoretical yield?
The maximum amount of product predicted from the reaction calculation.
What does percentage yield compare?
The actual amount of product with the maximum theoretical amount.
What is the percentage-yield formula?
percentage yield = (actual yield ÷ theoretical yield) × 100
Which quantity goes in the numerator of the percentage-yield calculation?
The actual amount of product obtained.
Which quantity goes in the denominator?
The maximum theoretical amount of product.
What does a 100% yield mean?
The actual amount obtained equals the theoretical maximum.
What does an 80% yield mean?
The actual amount obtained is 80% of the theoretical maximum.
Why can a reversible reaction reduce yield?
It may not go to completion, so not all reactants become the desired product.
How can separation reduce yield?
Some product can be lost while it is being separated or collected.
How can unexpected reactions reduce yield?
Some reactants form other products instead of the desired product.
Theoretical mass = 25 g and actual mass = 20 g. Percentage yield?
(20 ÷ 25) × 100 = 80%.
Theoretical mass = 60 g and actual mass = 45 g. Percentage yield?
(45 ÷ 60) × 100 = 75%.
Actual mass = 18 g and percentage yield = 90%. What theoretical mass would match this?
20 g, because 18 ÷ 20 × 100 = 90%.
Why should actual and theoretical amounts use compatible units?
Because they are being compared as a ratio.
Does a low percentage yield automatically tell you which loss mechanism occurred?
No. You need evidence about why less desired product was obtained.
Higher: what does a balanced equation provide for a theoretical-mass calculation?
The mole ratio between reactants and products.
Higher: after finding moles of product, how do you find theoretical mass?
Multiply moles of product by its relative formula mass in g mol⁻¹.
Higher: in 2Mg + O₂ → 2MgO, what is the mole ratio Mg:MgO?
1:1.
Higher: 12 g Mg is 0.50 mol. How many moles of MgO are theoretically formed?
0.50 mol, from the 1:1 Mg:MgO mole ratio.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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