2.1.3 (a) - The Mole

Syllabus

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) explanation and use of the terms:

(i) amount of substance

(ii) mole (symbol ‘mol’), as the unit for amount of substance

(iii) the Avogadro constant, N_A (the number of particles per mole, 6.02 × 10²³mol^–1)

(iv) molar mass (mass per mole, units g mol^–1),

(v) molar gas volume (gas volume per mole, units dm³ mol^–1)

What does this mean?

The Mole

Thinking about individual atoms/molecules in reactions is generally pointless as they are simply too small.

So we think about a standard amount of atoms/molecules called the mole.

This seems to scare GCSE students.

But it's no more unusual than measuring eggs by the dozen (12) except that the definition of a mole is a little complicated.

Mole Definition

A mole is the amount of substance containing as many atoms or molecules as there are Carbon atoms in 12g of ¹²C

This is frequently asked for in exams so learn it.

As discussed before, ¹²C is our standard atom. And 12g of it is precisely 1 mole.

It has taken over a hundred years to work out how many atoms that is.

The current estimate is around 602,214,141,070,409,084,099,072.

But this is probably not exactly correct and is anyway much more accurate than is required.

The A level definition is 6.02 x 10²³ (602,000,000,000,000,000,000,000)

We call this Avogadro's Number or must N_A.

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Molar Mass

You should already have learned about relative isotopic mass (for single isotopes), relative atomic mass (for single elements but with mixed isotopes), relative molecular mass (for simple covalent molecules) and relative formula mass (for ionic compounds and giant covalent ones).

They are all the masses of one mole of a substance.

So, rather than worrying about which one we are talking about we often simply use the term Molar Mass - to mean the mass of a mole of anything.

It is still in g mol^-1 (g/mol is for GCSE )

You will frequently be asked to use the molar mass to work out the number of moles in a particular mass of a substance.

From time to time you'll also be asked to work out a molar mass from a number of moles and a mass.

This is no different than it was at GCSE but if you've managed to forget, more later.

Molar Gas Volume

Avogadro never actually tried to calculate the number named after him.

He famously stated that "equal volumes of any gas contain the same number of molecules providing they are at the same temperature and pressure."

Unless otherwise stated in a question, we can assume that everything has been measured at Room Temperature and Pressure (RTP).

At which point it will occupy 24 dm³ - which is the same as 24 litres.

Or 24,000 cm³ - which is equal to 24,000 ml.

This is the Molar Gas Volume and is given on the data-sheet in the exam - you are not required to learn it.

Again, you probably covered this and the calculations you can do with it at GCSE.

And if not, more later.

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