Significance of the Mole

The Mole

Year 11

Syllabus dot points

• 2.2 c explore the concept of the mole and relate this to Avogadro’s constant to describe, calculate and manipulate masses, chemical amounts and numbers of particles in: (ACSCH007, ACSCH039)

– moles of elements and compounds 𝑛 = 𝑚 /𝑀𝑀 (n = chemical amount in moles, m = mass in grams, MM = molar mass in gmol-1 )

What we will cover​

• what is the mole?

• how chemists deal with numbers in chemistry

• ​conversions involving the mole

​Determining mass of individual atoms?

Back in the day:

• Gas Laws and chemical equations/ratios

  • gases react in certain proportions

  • development of stoichiometry

• Not very specific

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Now:

• We can determine the mass of ​individual atoms and molecules through mass spectrometry

  • ​Uses Newton's 2nd Law (we know the net force and acceleration of particles, and can simply solve for their mass)

  • It is more complicated than this - but this is the basis!

​The Mole

Using this method, it was determined that the mass of one carbon-12 atom is

1.933 x 10^-23g​

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So in 12g of carbon-12, there are 6.022 x 10²³ atoms (or 602 000 000 000 000 000 000 000)

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Therefore for titanium, which has a relative atomic mass of 48, 6.022 x 10²³ atoms is 48g

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For any element, ​the mass in grams that is equal to the relative atomic mass (that we find on the Periodic Table) has 6.022 x 10²³ contains atoms

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This extends to molecules, except the relative molecular mass instead of atomic mass (so 18g of water contains 6.022 x 10²³​

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Why do we need the mole?

• Atoms are so small, it is not logical to count them individually

• We therefore count them in groups, which we call moles

• Like counting eggs by the dozen or shoes in pairs

​Facts on The Mole

• The mole can refer to individual atoms, ions, molecules, or formula units

• ​Abbreviation is 'mol'

• ​Commonly used term is molar mass = mass of one mole of a substance

  • has the units of g/mol or g.mol^-1

• We can easily convert between mass, moles, and particles

• We can also easily calcul​ate mass, molar mass, or moles (as long as we have two of these numbers)

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Calculating number of particles, given ​moles (or inverse)

Multiple Choice

What is the mass of 4.6 x 10¹⁸ molecules of boric acid, H₃BO₃?

• 4.9 x 10^-6

• 4.7 x 10^-4

• 3.9 x 10^-17

• 7.7 x 10^-2

Multiple Choice

What mass of sulfur contains the same number of atoms as 63.55 grams of copper?

• 1 g

• 63.55 g

• 32.06 g

• 6.02 x 102³ g

​Calculating moles, given mass (or inverse)

Multiple Choice

Which has more moles, 20.18 g of neon or 39.95 g of argon?

• 20.18 g of neon

• 39.95 g of argon

• they are both 1 mole

• no way to predict the answer to this

Multiple Choice

How many molecules are there in 31.8 moles of water?

• 5.28 x 10^-23 molecules

• 1.91 x 10²⁵ molecules

• 5.28x 10^-25 molecules

• 1.91 x 10²³ molecules

​Next up

• Calculations: percentage composition, empirical formula, ​limiting reagent