Units

Temperature measured in degrees centigrade

A number without its correct units is meaningless. Students frequently forget to include the correct units with their answers. Such difficulties do not usually occur in the world of money. If you were offered 2000 for your second hand car, you might assume that you were being offered £2000. You might be disappointed to find that you were actually going to receive 2000p or even 2000 Polish Zlotys. In Chemistry exams missing or incorrect units are a frequent cause of lost marks. There are other benefits to being careful with units. Understanding units enables us to check our equations using a technique known as dimensional analysis. We can often work out how to calculate an answer if we know the units of that answer.

The following units are the ones we commonly meet in "A" Level Chemistry. Note that they may not be the same as the ones you use in your "A" Level Physics course. The standard unit for volume is the m³, but this is an inconveniently large unit for use in a standard chemical laboratory. Consequently, we tend to use the cm³ or dm³.

Quantity	Unit name	Unit symbol
Amount of substance	mole	mol
Concentration	mole per decimetre cubed	mol dm^-3 [also written M]
Density	gram per centimetre cubed	g cm^-3
Enthalpy change	kilojoule per mole	kJ mol^-1
Entropy	joule per mole per Kelvin	J mol^-1 K^-1
Heat	kilojoule	kJ
Mass	gram	g
Molar mass	gram per mole	g mol^-1
Pressure	atmosphere or Newton per metre squared [Pascal]	atm. or N m^-2 [Pa]
Specific Heat Capacity	joule per gram per Kelvin	J g^-1 K^-1
Temperature	degree centigrade (Celsius) or Kelvin	ºC or K
Volume	centimetre cubed [millilitre] or decimetre cubed [litre]	cm³ [ml] or dm³ [l]

A common error with units is to abbreviate mol to m, but this means metres! Students often also like to use mol dm^-3 when the volume referred to is not a dm^-3. If you are asked to work out the number of moles of ethanoic acid in a 750 ml bottle of wine, your answer should be in mol not mol dm^-3. Similarly, in a titration calculation you will be calculating numbers of moles. Only turn this into mol dm^-3 [M] if and when you are sure it is the number of moles in one decimetre cubed.

The decimetre cubed (dm³) and litre(l) are used interchangeably. There used to be a small difference in these units, but this has been changed and they are now identical.

Standard prefixes are used to change the size of the basic units. You should know the following prefixes shown in bold (the others are for information):

× 10³	kilo (k)	× 10^-3	milli (m)
× 10⁶	mega (M)	× 10^-6	micro (m)
× 10⁹	giga (G)	× 10^-9	nano (n)
× 10¹²	tera (T)	× 10^-12	pico (p)