Reaction calculations
The calculation route
- Most reaction calculations follow the same steps:
- change the known amount into moles,
- use the balanced equation ratio to find the moles you want,
- change back into mass, volume or concentration.
Limiting reactant & titrations
- The limiting reactant is the one that runs out first — it decides how much product forms (the other is in excess).
- In a titration you measure the volume of one solution that reacts with another; from volume and concentration you find the moles, then use the ratio.
Practice
The limiting reactant is the one that:
The limiting reactant is used up first, so it controls the amount of product; the other is in excess.
Formulae from data & percentages
- Empirical formula from masses/percentages: divide each by its $A_r$, then divide by the smallest.
- Percentage yield $= \dfrac{\text{actual}}{\text{theoretical}} \times 100$.
- Percentage purity $= \dfrac{\text{mass of pure substance}}{\text{mass of sample}} \times 100$.
Practice
A reaction could make 50 g of product but only 40 g forms. What is the percentage yield?
% yield = actual ÷ theoretical × 100 = 40 ÷ 50 × 100 = 80%.
Practice
To find an empirical formula from masses, you first:
Divide each mass (or %) by its Ar to get moles, then divide by the smallest to get the ratio.
You've got it
Key idea
- calculation route: mass → moles → (ratio) → moles → answer
- the limiting reactant runs out first and sets the product amount
- % yield = actual ÷ theoretical × 100; empirical formula = (mass ÷ $A_r$), then ÷ smallest