Mole Mountain
Count atoms by weighing them. Master the mole, the Avogadro constant and moles = mass / Mr, then use a balanced equation to find reacting masses.
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Counting by weighing ⚛️
Atoms are far too small and too many to count one by one. Chemists get around this with the **mole**, a fixed, huge number of particles that we can weigh out on a balance. Master the mole and you can predict exactly how much of each chemical reacts.
The mole and Avogadro 🔢
One **mole** of any substance contains the same number of particles: the **Avogadro constant**, **6.02 x 10^23** particles per mole. So one mole of carbon atoms and one mole of water molecules both contain 6.02 x 10^23 particles, even though they weigh different amounts.
How many particles?
One mole of any substance contains how many particles (the Avogadro constant)?
- 6.02 x 10^23
- 1000
- 100
- 6.02 x 10^11
Relative formula mass 📊
The **relative formula mass (Mr)** of a substance is the total of the **relative atomic masses (Ar)** of all its atoms. For water, H2O: (2 x 1) + 16 = **18**. The molar mass is the Mr in grams, so one mole of water weighs 18 g.
Work out the Mr
An interactive activity.
The key equation 📐
The equation that unlocks everything is: **number of moles = mass / Mr** Rearranged, **mass = moles x Mr**. Keep the units straight: mass in grams, Mr with no units, moles in mol.
How many moles?
An interactive activity.
What is the mass?
An interactive activity.
Reacting masses 🔀
A **balanced equation** tells you the ratio in which substances react. The big numbers in front (the coefficients) are a **mole ratio**. For example, in N2 + 3H2 -> 2NH3, one mole of nitrogen reacts with three moles of hydrogen to make two moles of ammonia.
The reacting-mass method
An interactive activity.
Use the ratio
An interactive activity.
Match each quantity to how you find it
- Number of moles
- Mass
- Mr of a compound
- Avogadro constant
- mass / Mr
- moles x Mr
- Add up the Ar of every atom
- 6.02 x 10^23 per mole
Rearrange it
You know the number of moles and the Mr. How do you find the mass?
- mass = moles x Mr
- mass = moles / Mr
- mass = Mr / moles
- mass = moles + Mr
Mole summary
One mole contains 6.02 x 10^23 particles, the _____ constant. The number of moles = mass _____ Mr. To go the other way, mass = moles _____ Mr. In a balanced equation, the big numbers give the mole _____.