What is a Mole and Avogadro’s Number in Chemistry

Core Concepts – Mole in Chemistry

So what is a mole? In this tutorial, you will learn the definition of the mole, an important unit of measurement in chemistry. You will also learn about Avogadro’s number and why it is useful. From here, you will be able to use these concepts in chemistry problem-solving.

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Definition of the Mole and Avogadro’s Number

The mole, or “mol” is a unit of measurement in chemistry, used to designate a very large number of molecules, atoms, or particles. This very large number is called Avogadro’s Number: 6.02214 x 10²³, the number of units in a mole. These numbers are very important for telling us about the quantity of elements. A mole of a substance will end up being anywhere from a few grams of a substance, to hundreds of grams.

Applications of the Mole

A mole of an element

Let’s look at a mole of an element. In a single mol, regardless of the element, there are 6.02214 x 10²³ atoms. This means that 1 mol of hydrogen contains 6.02214 x 10²³ hydrogen atoms and 1 mol of chlorine contains 6.02214 x 10²³ chlorine atoms. The mole has a ton of applications, throughout all levels of chemistry, from laboratory work to balancing chemical equations. Let’s take a look at it in action.

A mole of a compound

Let’s look at a mole of a compound, sodium chloride – NaCl. A mole of NaCl will have Avogadro’s number of molecules of sodium chloride it in. A mole of sodium chloride contains a mole of sodium atoms and a mole of chlorine atoms.

The Mole in Elements, Molecules, and Compounds

If an element is written with a coefficient, that coefficient tells you the number of moles of that element is present. For example, writing 2C, means that there are 2 mols of Carbon. In other words, there are 2 moles of carbon atoms. We already know that one mole of an element contains 6.02214 x 10²³ atoms (Avogadro’s number), so two moles are the same as 2 * 6.02214 x 10²³ atoms = 12.04428 x 10²³ carbon atoms. This idea of the coefficients will come up a lot in chemical equations.

If in a given molecule or compound, the element has a subscript, the subscript tells you the moles of the element in one mole of the molecule. Note the subtle distinction between moles of a molecule, and moles of an element within a molecule. The number of moles of an element can be different than the moles of that element in a larger molecule. This may seems confusing at first glance, but looking at some examples will help.

Examples of Understanding Moles in Chemistry

Refer to the molecule O₂

Since the coefficient of the compound is 1, we are working with 1 mole of the molecule O₂
- 1 mole of O₂ = 6.02214 x 10²³ molecules O₂
Since the atom O has a subscript 2, this means that there are two moles of O atoms in the molecule
- 2 moles of O = 2 * 6.02214 x 10²³ atoms = 12.04428 x 10²³ oxygen atoms

2. Let’s try analysis of a compound: CH₄

Since the coefficient is 1, we are working with 1 mole of the compound CH₄
- 1 mole of CH₄ = 6.02214 x 10²³ molecules CH₄
Within the compound there is 1 mole of C, since the subscript is 1
- 1 mole of C = 6.02214 x 10²³ atoms C
Within the compound there are 4 moles of H, since the subscript is 4
- 4 moles of H = 4 * 6.02214 x 10²³ atoms = 24.08856 x 10²³ atoms H

3. Here’s one more that’s challenging: 3NaCl

Since the coefficient of the compound is 3, we are working with 3 moles of the compound NaCl
- 3 moles of NaCl = 3 * 6.02214 x 10²³ molecules NaCl
Within the compound there are 3 mol Na (the coefficient gets distributed to the atoms as well!)
- 3 moles of NaCl = 3 * 6.02214 x 10²³ = 18.0663 x 10²³ atoms of Na
Within the compound there are 3 mol Cl
- 3 moles of NaCl = 3 * 6.02214 x 10²³ = 18.0663 x 10²³ atoms of Cl

Converting from Grams to Moles

It is also possible to convert from grams to moles and vice versa using an element or compound’s molecular weight. Recall the the molecular weight of a compound is the sum of the molecular weight of its elemental components.

When given the moles of a compound, to find the grams, multiply by molecular weight.
When given the grams of a compound, to find the moles, divide by molecular weight.

To find the atoms of a molecule given the grams, it is first necessary to convert to moles using molecular weight, and then to atoms with Avogadro’s number.

Examples of Gram Conversions

How many grams are in 9.2 moles of NO₂?

The molecular weight of NO₂ is 46 g/mol
To find the grams in 1.2 moles, divide by the molecular weight!
- 9.2/46 = 0.2 grams NO₂

2. How many atoms are in 1.5 grams of CO?

First convert the grams of CO to moles using molecular weight
- The molecular weight of CO is 28 g/mol
- Moles CO = 1.5 * 28 g/mol = 42 moles
To find the number of atoms, multiply the number of moles by Avogadro’s number
- Atoms CO = 42 moles * 6.02214 x 10²³ = 252.93 x 10²³ atoms CO

Video Tutorial on Basic Conversions: Molar Mass, Moles, and Atoms

Please enjoy this conversions tutorial for extra practice problems and topic explanation. In this video, you will learn about moles, molar mass, and converting between grams, atoms, and moles.