What is a Mole and Avogadro’s Number in Chemistry

mole definition

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

  1. 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.

what is a mole in chemistry?

Examples of Gram Conversions

  1. 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

Further Reading

Thanks for learning what is a mole in chemistry, and about Avogadro’s number. For more general chemistry topics, see the articles below!

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