3.9

# Moles, mass and concentration

Students should become familiar with the calculations required, but it can be quite challenging for them to apply them, especially as we often have to convert units (say from cm3 to dm3). It might be worth reminding yourself of some of the key mathematical manipulations. There are some examples on the STEM Resources website which you can use for your own revision or as practice for your own students

Key things to remember are:

The mole is a measure of the number of things- in this case particles (atoms, molecules, ions, etc.). One mole is 6.022 x 1023 particles; this is called Avagadro’s number and is huge. The mole is a much more convenient unit than actually counting particles (which can’t really be done!).

Molar mass (or atomic mass for elements) has a number of names: relative formula mass, relative atomic mass, etc. They all have the same meaning: the mass (in grams) of 1 mole of that substance.

The unit of molar mass is grams per mol, or g/mol (or gmol-1)

The link between moles, mass and molar mass is:

Mass = moles x molar mass

Students should be able to rearrange this formula to be able to calculate an unknown, given the two other factors. Use the language for molar mass that is mentioned in your teaching specification, to avoid possible confusion for students.

Concentration is the number of moles of a substance dissolved in one decimetre cubed (dm3) which is 1000cm3 or 1 litre, usually in pure water.

Concentration = moles / volume

Concentration is given in mol/dm3, properly written as moldm-3 but this notation is often not used until post-16 level.

moldm-3 is often abbreviated to M, as it is easier to write, especially labelling a bottle, but for assessments, students should be encouraged to use the full notation.

Students often get confused converting between different units of volume or mass, and it is worth incorporating such conversions within practical work to explore students understanding.

## Comment

Whilst mathematical calculations could be practiced without undertaking practical work. What do you think are the advantages of combining mathematics with practical science activities?