MYP Chemistry Topic The Mole Concept and Chemical…

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The Mole – Counting the Uncountable

Atoms and molecules are tiny – far too small to count individually.
Chemists needed a way to:
1. talk about numbers of particles
2. but work with measurable amounts like grams and litres.
That’s why we use the mole.

What is a mole?

Definition

Mole

The amount of substance that contains $N_A=6.022×10^{23}$ particles (atoms, molecules, ions, etc.).

That big number in the definition of the mole is called Avogadro’s constant.

Analogy

Think of a mole as a chemist’s dozen:

A dozen = 12 of something
A mole = $6.022×10^{23}$ of something

Example

1 mol of carbon atoms = $6.022×10^{23}$ $C$ atoms
1 mol of water molecules = $6.022×10^{23}$ $H_2O$ molecules
This is how we connect the microscopic world (particles) to the macroscopic world (mass, volume).

Molar Mass – Linking Moles to Mass

Using the Periodic Table to Find Ar and Mr

To use the mole in calculations, you start with the periodic table.
On a typical periodic table each element has:
1. Atomic number (Z) – number of protons
2. Relative atomic mass (Aᵣ) – the average mass of its atoms, compared to carbon-12
We use Aᵣ values to calculate relative molecular mass (Mᵣ) and molar mass.

Example

Step 1 – Pick up Aᵣ from the periodic table

The most frequently used values:

H ≈ 1
C ≈ 12
O ≈ 16
Na ≈ 23
Cl ≈ 35.5
Ca ≈ 40
N ≈ 14

These numbers have no units when used as Aᵣ, but the molar mass has units of g mol⁻¹ and is numerically equal to Mᵣ.

Step 2 – Use Aᵣ to find Mᵣ (relative molecular / formula mass)

For molecules, add up the Aᵣ of all atoms in the formula
For ionic compounds, do the same (it’s often called “relative formula mass” instead)

Water – H₂O

$$M_r\left(\mathrm{H}_2 \mathrm{O}\right)=2 \times A_r(\mathrm{H})+A_r(\mathrm{O})=2 \times 1+16=18$$

So molar mass = 18 g mol⁻¹

Sodium chloride – NaCl

$$M_r(\mathrm{NaCl})=A_r(\mathrm{Na})+A_r(\mathrm{Cl})=23+35.5=58.5$$

Molar mass = 58.5 g mol⁻¹

Calcium hydroxide – Ca(OH)₂

Be careful with brackets – (OH)₂ means two OH groups:

$$M_r\left(\mathrm{Ca}(\mathrm{OH})_2\right)=A_r(\mathrm{Ca})+2 \times\left[A_r(\mathrm{O})+A_r(\mathrm{H})\right]=40+2 \times(16+1)=$$ $$=40+2 \times 17=40+34=74$$

Molar mass = 74 g mol⁻¹

Diatomic gases
Some elements exist as diatomic molecules in nature: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Example for O₂: $$M_r\left(\mathrm{O}_2\right)=2 \times 16=32$$

Tip

When you see O₂, H₂, Cl₂ in an equation, always use the molecular Mr, not the atomic Ar.

Molar Mass

The molar mass of a substance is the mass of 1 mole of that substance.
Unit: g mol⁻¹
Numerically equal to the relative atomic/molecular mass on the periodic table.

Example

Carbon (C): Ar ≈ 12.0 → molar mass = 12.0 g mol⁻¹
Water (H₂O): $$M_r=2 \times 1.0+16.0=18.0 \Rightarrow \text { molar mass }=18.0 \mathrm{~g} \mathrm{~mol}^{-1}$$

Key formula

$$n=\frac{m}{M}$$

Where:

$n$ = moles (mol)
$m$ = mass (g)
$M$ = molar mass (g mol⁻¹)

Example question

How many moles are in 36.0 g of water?

Solution

$$M\left(\mathrm{H}_2 \mathrm{O}\right)=18.0 \mathrm{~g} \mathrm{~mol}^{-1} \Rightarrow n=\frac{36.0}{18.0}=2.0 \mathrm{~mol}$$

Example question

What mass is 2.0 mol of H₂O?

Solution

$$m=n M=2.0 \times 18.0=36.0 \mathrm{~g}$$

Particles and Moles

Once you know the moles, you can find the number of particles using Avogadro’s constant: $$N=n \times N_A$$

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MYP Chemistry The Mole Concept and Chemical Calculations Notes

The Mole – Counting the Uncountable

What is a mole?

Molar Mass – Linking Moles to Mass

Using the Periodic Table to Find Ar and Mr

Step 1 – Pick up Aᵣ from the periodic table

Step 2 – Use Aᵣ to find Mᵣ (relative molecular / formula mass)

Molar Mass

Key formula

Particles and Moles

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Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account

The Mole – Counting the Uncountable

What is a mole?

Molar Mass – Linking Moles to Mass

Using the Periodic Table to Find Ar and Mr

Step 1 – Pick up Aᵣ from the periodic table

Step 2 – Use Aᵣ to find Mᵣ (relative molecular / formula mass)

Molar Mass

Key formula

Particles and Moles

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Periodic table5 subtopics

Matter5 subtopics

Types of chemical reaction18 subtopics

International Union of Pure and Applied Chemistry (IUPAC)4 subtopics

Pure and impure substances2 subtopics

The atmosphere2 subtopics

Bonding4 subtopics

MYP Chemistry The Mole Concept and Chemical Calculations Notes

Periodic table5 subtopics

Matter5 subtopics

Types of chemical reaction18 subtopics

International Union of Pure and Applied Chemistry (IUPAC)4 subtopics

Pure and impure substances2 subtopics

The atmosphere2 subtopics

Bonding4 subtopics

The Mole – Counting the Uncountable

What is a mole?

Molar Mass – Linking Moles to Mass

Using the Periodic Table to Find Ar and Mr

Step 1 – Pick up Aᵣ from the periodic table

Step 2 – Use Aᵣ to find Mᵣ (relative molecular / formula mass)

Molar Mass

Key formula

Particles and Moles

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account

The Mole – Counting the Uncountable

What is a mole?

Molar Mass – Linking Moles to Mass

Using the Periodic Table to Find Ar and Mr

Step 1 – Pick up Aᵣ from the periodic table

Step 2 – Use Aᵣ to find Mᵣ (relative molecular / formula mass)

Molar Mass

Key formula

Particles and Moles

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account