Reacting masses and yield
| English | Chinese | Pinyin |
|---|---|---|
| mole ratio | 摩尔比 | mó ěr bǐ |
| stoichiometry | 化学计量 | huà xué jì liàng |
| percentage yield | 产率 | chǎn lǜ |
| excess reagent | 过量试剂 | guò liàng shì jì |
| limiting reagent | 限量试剂 | xiàn liàng shì jì |
How much product will you get?
- The numbers in a balanced equation give the mole ratio 摩尔比 — the stoichiometry 化学计量.
- From it we work out reacting masses.
- Real reactions give less than the maximum, and one reactant runs out first.
Reacting mass route
Follow a balanced equation from known mass to predicted product mass.
Percentage yield is the actual yield divided by the theoretical yield, times 100.
It measures how much product you actually obtained compared with the maximum possible.
The stoichiometry of a reaction comes from:
The balancing numbers give the mole ratio in which species react and form.
The reacting-mass method
To find a reacting mass:
- change the known mass → moles ($n = m/M$).
- use the mole ratio to find the moles you want.
- change those moles → mass.
Put the steps of a reacting-mass calculation in order.
mass → moles → ratio → moles → mass. The mole ratio is the bridge between the two substances.
In 2H₂ + O₂ → 2H₂O, how many moles of O₂ are needed to react with 4 mol of H₂?
The ratio H₂ : O₂ is 2 : 1, so 4 mol H₂ needs 4 ÷ 2 = 2 mol O₂.
Percentage yield 产率
- You usually get less product than the maximum.
A reaction makes 8 g of product when the maximum possible is 10 g. What is the percentage yield?
percentage yield = (actual / maximum) × 100 = (8 / 10) × 100 = 80%.
Limiting and excess reagent 过量试剂
- The limiting reagent 限量试剂 runs out first and decides how much product forms.
- The excess reagent is left over.
- To find it: divide each reactant's moles by its number in the equation — the smallest result is limiting.
The reactant that runs out first (the limiting reagent) sets how much product forms.
The limiting reagent in a reaction is the one that:
The limiting reagent is used up first, so it sets the amount of product; the other is in excess.
The reactant that runs out first and decides how much product forms is the ______ reagent.
The other reactant is "in excess" — some of it is left over.
You've got it
- stoichiometry = mole ratio from the balanced equation
- reacting mass: mass → moles → ratio → moles → mass
- $\text{percentage yield} = \dfrac{\text{actual}}{\text{maximum}} \times 100\%$
- the limiting reagent runs out first and sets the product; base the calculation on it