Percentage Yield Cannot Exceed 100%

What students typically write

“My percentage yield is 115%, which shows the reaction was very efficient.” Or students present a value above 100% without questioning it.

When students calculate a yield above 100% they often submit it as if it were a valid result, not recognising that it signals an experimental or calculation error.

Why it is wrong

Percentage yield = (actual yield / theoretical yield) x 100.

The theoretical yield is the maximum mass of product that could form if all the limiting reagent were converted to product with no losses. It is a ceiling. You cannot produce more product than the reactants allow, just as you cannot bake more cakes than your ingredients permit.

In real experiments, yield is always less than 100% because of:

• Transfer losses (product left on glassware)
• Incomplete reactions (not all reactant converts)
• Side reactions (reactants forming unwanted products)
• Purification losses (product lost during filtration, washing, recrystallisation)

A value above 100% therefore does not mean the reaction was efficient. It means something went wrong.

What the mark scheme actually wants

A typical question: “A student prepared copper sulfate crystals and calculated a percentage yield of 108%. Suggest one reason why the yield appears to be greater than 100%.” [1]

Accepted answers:

• The crystals were not fully dried / contained water (1 mark)
• Impurities were present in the product (1 mark)

A 6-mark evaluation question might ask for the reason and a suggestion for improvement: dry the crystals thoroughly in a warm oven and reweigh until constant mass is achieved.

Worked example: wrong vs right

Question: A student reacted 10.0 g of CaCO3 with excess HCl and obtained 6.2 g of CaCl2. Calculate the percentage yield. [Mr: CaCO3 = 100, CaCl2 = 111] [3]

Wrong approach (leading to over 100%): If the student uses Mr of CaCl2 = 95 by mistake: Theoretical yield = (10.0/100) x 95 = 9.5 g. Yield = (6.2/9.5) x 100 = 65.3%. This is under 100% but wrong because Mr is wrong.

Right approach: Moles of CaCO3 = 10.0 / 100 = 0.10 mol (1). From the equation, moles of CaCl2 = 0.10 mol, so theoretical yield = 0.10 x 111 = 11.1 g (1). Percentage yield = (6.2 / 11.1) x 100 = 55.9% (1). This scores 3/3 and gives a sensible value below 100%.

How to avoid this mistake

If your calculated percentage yield is above 100%, do not submit it. Instead, check three things:

1. Is your theoretical yield correct? Recalculate Mr values and check the mole ratio from the balanced equation.
2. Is your actual yield accurate? Check whether the product was fully dried and free of impurities.
3. Did you use the correct masses? Ensure you subtracted the mass of the container.

In a calculation (no experimental data), a value above 100% always means a maths error. In a practical question, it means impure or wet product. Either way, flag it and explain why.

For the full method of calculating percentage yield and percentage purity, see percentage yield and purity. Reacting mass calculations, which provide the theoretical yield, are covered at reacting mass calculations.