# Amount of Substance- The Mole and The Avogadro Constant (A-Level Chemistry)

# The Mole and The Avogadro Constant

## The Mole and Avogadro Constant

### Definitions

The **mole **is a unit used to measure the amount of substance. One mole of a substance contains as many substance units as there are atoms in 12g of carbon-12. A mole is often written as mol in the shorthand form.

**Avogadro’s constant** is 6.02 x 1023 atoms. This number is the number of particles (atoms, molecules, ions, electrons) found in one mole.

### Calculations

It is helpful to learn the following equation as it helps to adjust your calculation depending on which pieces of information you are given:

Let’s look at an example which involves this equation:

**Practice Question: **How many atoms are present in 0.34 moles of sodium metal?

Number of particles = Number of moles x Avogadro’s constant

Particle = Na atom

Number of moles = 0.34

Avogadro’s constant = 6.02 x 1023

Therefore the number of atoms = 0.34 x 6.02 x 1023 = 2.05 x 1023 Na atoms.

## Amount of substance

### Mass and Relative Molecular Mass (Mr)

The mass of 1 mole of a substance is the same as the relative molecular mass of the substance in grams.

For example, the relative molecular mass of carbon dioxide (CO2) is 44. This means that 1 mole of carbon dioxide has a mass of 44g.

### Formula Triangles

When you have an equation that you need to remember, it useful to draw yourself a formula triangle to help you visualise the calculation you need to do.

To calculate the number of moles you can use the following equation:

Here is a formula triangle which helps you visualise the equation. Cover up the piece of information you are not given in the question to help you rearrange the equation.

**Example: **What is the mass of 2 moles of carbon dioxide CO2 ? (RAM are C = 12, O

Mass of substance = ?

Number of Moles = 2

Mr of substance = 12 + (16 x 2) = 44

Mass of substance = number of moles x Mr = 2 x 44 = 88g

**→What is the mole?**

The mole is a unit of measurement in chemistry that is used to express the amount of a substance. It is defined as the amount of a substance that contains the same number of entities as there are in 12 grams of pure carbon-12.

**→What is the Avogadro constant?**

The Avogadro constant is the number of entities in one mole of a substance. It is defined as 6.022 x 10^23 entities per mole.

**→Why is the mole important in chemistry?**

The mole is important in chemistry because it provides a way to convert between the mass of a substance and the number of entities in that substance. This allows chemists to make accurate and precise measurements of the amount of a substance in a sample.

**→How does the Avogadro constant relate to the mole?**

The Avogadro constant is directly related to the mole. The mole is defined as the amount of a substance that contains the same number of entities as there are in the Avogadro constant.

**→Can the Avogadro constant be measured directly?**

The Avogadro constant cannot be measured directly. Instead, it is determined by indirect methods, such as X-ray diffraction or electronic spectroscopy.

**→What is the significance of the Avogadro constant in chemistry?**

The Avogadro constant is significant in chemistry because it provides a way to convert between the number of entities in a sample and the mass of that sample. This is essential for making accurate and precise measurements of the amount of a substance in a sample.

**→How can I use the mole and Avogadro constant in A-Level Chemistry?**

In A-Level Chemistry, you can use the mole and Avogadro constant to calculate the number of entities in a sample of a substance, as well as to convert between the mass of a substance and the number of entities in that substance. This will help you understand the concepts of stoichiometry, chemical reactions, and the mole.

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