Reaction calculations
Introduced| English | Meaning |
|---|---|
| mole ratio/məʊl ˈreɪʃɪəʊ/ | The ratio of amounts of substances measured in moles, often given by the coefficients in a balanced equation. |
| yields/jiːldz/ | The amount of product obtained in a reaction, often expressed as a percentage of the theoretical amount. |
Predicting how much you get
- A balanced equation is a recipe: it tells you exactly how much product a reaction makes.
- Reaction calculations use moles to work this out.
Balance the equation
Step the coefficients until every element has the same number of atoms on both sides.
Reaction calculation route
Follow a known mass through a balanced equation to the answer.
How many moles are in 4 g of hydrogen gas (Mr = 2)?
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moles = 4 ÷ 2 = 2.
The big numbers in a balanced equation give the ______ ratio.
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Reactions combine in whole-number mole ratios.
The mole ratio 摩尔比
- The big numbers in a balanced equation give the mole ratio of reactants and products.
- In 2H₂ + O₂ → 2H₂O, 2 moles of hydrogen make 2 moles of water.
A titration measures exactly how much acid reacts with a base
In 2H₂ + O₂ → 2H₂O, the mole ratio of H₂ to H₂O is:
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2 moles of H₂ give 2 moles of water.
Put the reaction-calculation steps in order.
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Mass → moles → ratio → moles → mass.
The method
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- Find the moles of the substance you know (mass ÷ Mr).
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- Use the mole ratio to find moles of the substance you want.
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- Convert back to mass (moles × Mr) if needed.
Worked example. How much water from 4 g of hydrogen (Mr = 2) burning in oxygen? Moles of H₂ = 4 ÷ 2 = 2. The ratio H₂ : H₂O is 2 : 2, so 2 moles of water form. Mass = 2 × 18 = 36 g.
If 2 moles of water form (Mr of water = 18), what mass of water is that?
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mass = 2 × 18 = 36 g.
Why it matters
- These calculations let chemists and factories work out yields 产率 and amounts needed.
- They avoid waste and predict exactly how much product to expect.
Use the mole ratio, not the mass ratio. Reactions combine in whole-number ratios of moles, not grams. Always convert mass to moles before applying the ratio from the equation.
You've got it
- the balanced equation gives the mole ratio of reactants to products
- method: mass → moles → (ratio) → moles → mass
- always work in moles, not masses, when using the ratio