# Explain why the percentage yield of a reaction is almost always less than 100%, referring to at least two distinct reasons.

> IB DP Chemistry Standard Level (2023 syllabus) — R2.1 How much? The amount of chemical change · Explain · 4 marks

## Mark scheme (4 marks)

1. The reaction may be reversible / may not go to completion, so not all reactants are converted to products.
2. Side reactions / competing reactions consume some of the reactants, producing by-products instead of the desired product.
3. Product is lost during practical steps such as transferring, filtering, or purifying, reducing the amount collected.
4. The theoretical yield assumes complete conversion of the limiting reagent to product, but real conditions rarely achieve this ideal.

## Key terms

- [percentage yield](https://www.gradenine.co.uk/glossary/percentage-yield)

## Related

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