The Mole and Mole Calculations
The mole is a key idea in Chemistry. It lets us count particles (like atoms or molecules) using mass, just like we count eggs in dozens.
What is a mole?
- One mole of a substance contains 6.022 × 10²³ particles
- This number is called Avogadro’s constant
- A mole can mean atoms, molecules, ions, or formula units
💡 1 mole of any substance = its Mr in grams
The mole triangle
To calculate moles, use the triangle below:
n = No. Moles
m = Mass (g)
Mr = Relative formula Mass
Formulas:
- Moles = mass ÷ Mr
- Mass = moles × Mr
- Mr = mass ÷ moles
Example 1: moles from mass
Question: What is the number of moles in 18 g of H₂O? (Mr = 18)
Answer:
Moles = 18 ÷ 18 = 1 mole
Example 2: mass from moles
Question: What is the mass of 2 moles of CO₂? (Mr = 44)
Answer:
Mass = 2 × 44 = 88 g
Example 3: Mr from mass and moles
Question: If 0.5 moles of a substance has a mass of 20 g, what is its Mr?
Answer:
Mr = 20 ÷ 0.5 = 40
Why is the mole useful?
- It links mass to the number of particles
- Helps with balanced equations and reaction amounts
- Essential for calculating yields and limiting reactants
Questions
- What is Avogadro’s constant?
- What is the mass of 1 mole of H₂O?
- What is the formula to calculate moles?
- How many moles are in 88 g of CO₂? (Mr = 44)
- What does 1 mole represent?
Summary
- A mole is a quantity that contains 6.022 × 10²³ particles
- The mass of 1 mole = Mr in grams
- Use the formula: moles = mass ÷ Mr
- The mole is used to calculate amounts in reactions
- It links mass, particles, and balanced equations