Reacting masses and yield
Reacting masses
- The numbers in a balanced equation give the mole ratio — the stoichiometry.
- From it we work out reacting masses.
- Real reactions give less than the maximum, and one reactant runs out first.
Practice
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.
Practice
The correct order for a reacting-mass calculation is:
Convert the known mass to moles, apply the ratio, then convert back to mass.
Percentage yield
$$\text{percentage yield} = \frac{\text{actual amount of product}}{\text{maximum possible}} \times 100\%$$
- You usually get less product than the maximum.
Practice
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
- The limiting reagent 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.
Practice
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.
You've got it
Key idea
- 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