Reacting Masses
Lajoy Tucker
Teacher
Contents
Introduction and Definitions
Reacting Masses: Calculations used to determine how much of a reactant is required or how much product is formed in a chemical reaction.
or M (relative formula mass or molar mass) :The mass of 1 mole of a substance
Basic Principles
Main Formulae:
Moles = Mass () /
Steps for Reacting Mass Calculations:
1. Write a balanced equation.
2. Find the moles of the known substance using
3. Use mole ratios from the equation to find the moles of the unknown substance.
4. Convert moles to mass using
5. May also have to use for reactions involving solutions.
Why Mole Ratios Matter
The coefficients in a balanced chemical equation tell you the ratio of moles of reactants and products. These ratios must be used when converting between substances.
Key Assumptions
Reactions go to completion.
All reagents are pure.
You are calculating the theoretical mass which is the maximum mass you could make (not actual yield).
Worked Examples
Example 1: Mass of Product from Given Reactant
What mass of water is produced when 6.00 g of hydrogen reacts with oxygen?
Answer:
Step 1: Moles of :
Step 2: Mole ratio makes
Step 3: Mass of :
of
Example 2: Mass of Reactant Needed
What is the minimum mass of iron required to react completely with 7.0 moles of oxygen gas.
Answer:
Moles of
Mole ratio moles of
of
Mass of (2s.f)
Example 3: Limiting Reagent
of reacts with of Oxygen. What mass of aluminium oxide is formed?
Answer:
Moles of
Moles of
Required mole ratio: divide both moles by coefficients:
Al is therefore the limiting reagent.
Moles of formed: is in a ratio
Moles of
(3s.f)
Note - The appropriate number of significant figures for an answer is determined by the least number of significant figures given in the question.
No answer provided.
Practice Questions
Question 1
How many grams of are produced when of propane () burns in excess oxygen?
of
Moles
Mole ratio:
Moles of
Mass of (2s.f)
Answer:
Question 2
What mass of Iron (III) oxide is needed to react with of Aluminium?
Moles of
Mole ratio:
Moles of
of
Mass (3s.f.)
Answer: