Moles

Just like the dozen the mole is just a number. It is a huge number, 602200000000000000000000 to be exact (Normally written as ), but it is just a number and it is called Avogadro’s Number.

Its use is very simple, it helps chemists change the RAM or RMM of any substance into grams. What this means is that 1 mole of substance would have the equivalent weight in grams as the atomic mass written on the periodic table.

For example:

1 mole of C = 12 g

1 mole of O₂ = 32g

1 mole of Na = 23g

It is very important to note that the mass of each individual atom is taken into consideration, and as such O₂ would be equal to 2*16 as there are two atoms of Oxygen.

Example 1

What is the mass of 0.1 moles of BaSO₄?
This question can be answered in two steps. First by finding the mass of one mole of BaSO₄ and then by finding the mass of 0.1 moles of BaSO₄.

Step 1

1 mole of  BaSO₄ contains 1 Ba, 1 S and 4 O.

Ba = 137 grams per mole

S = 32 grams per mole

O = 16 grams per mole

This would mean that BaSO₄ would weigh 137 + 32 + 16*4 = 233 grams per mole

Step 2

1 mole of BaSO₄ = 233g

0.1 moles of BaSO₄ = ?

This would mean that 0.1 moles of BaSO₄ weighs 23.3g.

Example 2

What would be the mass of 1 mole of the compound when 0.15 moles weighs 30g?

1 mole = ?

0.15 moles = 30g

1 mole of this compound would weigh 200g.

Solutions

The second aspect of the mole is the use of moles in concentration. The concentration can be defined as the amount of substance in a specific volume, and in chemistry this is no exception.

The unit for volume is the dm³ or the litre, and therefore molarity/concentration would be equal to the number of moles in 1L. This is the reason why molarity can be opened up to the number of moles:1000mL, using simple proportion instead of using the equation.

Example 3

What is the number of moles in 200mL of 0.2M HCl?

0.2M means 0.2 moles in 1000mL

∴ 0.2 moles = 1000mL

? moles = 200mL

0.04 moles are present in 200mL of 0.2M HCl solution.

Example 4

What is the concentration when 1.4g of NaCl is dissolved in 75mL?

Mass of 1 mole NaCl = 23g + 35.3g = 58.5g

1 mole = 58.5g

? moles = 1.4g

0.0239 moles of NaCl were dissolved in 75mL.

? moles = 1000mL

0.0239moles = 75mL

The concentration of 1.4g NaCl in 75mL is equal to 0.319M.

Gases

The last aspect of moles considers gases. These are discussed in a chapter specific for gases.

The two most important things are that 1 mole of gas has a constant value, 24dm at r.t.p. and 22.4dm at s.t.p and the relationship for gases that is PV=nRT.

Tips

When working a question about moles realise that only the moles of substances can be compared with each other. Thus it becomes important to work out which substances are important and which moles values to find.

In order to be able to plan on what values are needed a table such as the one below:

	A	B	-->	C	D
mass
volume
molarity
moles

Questions on Moles