The mole and the Avogadro constant
Introduced| English | Meaning |
|---|---|
| mole/məʊl/ | The SI unit of amount of substance, containing exactly 6.02214076 times ten to the power 23 specified entities. |
| Avogadro constant/ˌævəˈɡædrəʊ ˈkɒnstənt/ | The number of specified elementary entities in one mole, exactly 6.02214076 times ten to the power 23 per mole. |
Counting atoms by weighing
- You can't count atoms one by one — there are trillions in a speck of dust.
- So chemists count them in huge batches called moles 摩尔.
Mole particle lab
particles = n x Avogadro constant
Change moles and see particle number scale with Avogadro constant.
The number of particles in one mole is called the ______ constant.
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It is about 6.02 × 10²³.
The mole
- A mole is a fixed, enormous number of particles: the Avogadro constant 阿伏伽德罗常数 (6.02 × 10²³).
- One mole of any substance contains that many particles.
Convert between moles and mass, gas volume, concentration or number of particles
How many moles are in 36 g of water (Mr = 18)?
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moles = 36 ÷ 18 = 2.
How many moles are in 88 g of CO₂ (Mr = 44)?
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moles = 88 ÷ 44 = 2.
The number of moles is calculated as:
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moles = mass ÷ Mr.
Moles, mass and Mr
- The mass of one mole (in grams) equals the substance's Mr.
- moles = mass ÷ Mr — the key equation for counting by weighing.
Worked example. How many moles are in 36 g of water (Mr = 18)? moles = 36 ÷ 18 = 2 moles. So 36 g of water contains 2 × 6.02 × 10²³ molecules.
What is the mass of 3 moles of water (Mr = 18)?
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mass = moles × Mr = 3 × 18 = 54 g.
Going both ways
- To find mass from moles: mass = moles × Mr.
- This lets you convert between grams (what you weigh) and moles (what you count).
Mass and moles are not the same. 1 mole of carbon (12 g) and 1 mole of magnesium (24 g) have the same number of atoms but different masses. Always divide by the correct Mr.
You've got it
- a mole = 6.02 × 10²³ particles (the Avogadro constant)
- moles = mass ÷ Mr; the mass of one mole in grams equals the Mr
- rearrange: mass = moles × Mr to convert back