General Chemistry/Measurement

Moles

The mole is a unit used to measure the number of particles in a substance (where particles can be atoms, molecules, electrons, ions etc.). One mole of something contains 6.02205 × 10²³ particles. This rather large quantity is a constant called Avogadro's Number, which is abbreviated as N_A. It simply refers to the number of particles in one mole.

$N_A = 6.02214 \times 10^{23}\hbox{ mol}^{-1}$

For example, one mole of helium gas contains 6.022×10²³ atoms; 2 moles of oxygen gas contains 1.204×10²⁴ molecules.

Moles and Mass

It would be useful to be able to convert between the number of moles and the mass of a substance. The molar mass is the mass of one mole of a substance, measured in grams per mole or $\hbox{g}\cdot\hbox{mol}^{-1}$. Not by coincidence, the molar mass is the same as the atomic weight of a substance (this is how the concept of moles was originally defined).

For example, the molar mass (and atomic weight) of carbon-12 is 12 g·mol^-1, so 1 mole of carbon-12 weighs 12 grams. Similarly, chlorine has a molar mass of 35.45 g·mol^-1, so one mole of chlorine atoms weighs 35.45 g. There are two chlorine atoms in one molecule of Cl₂ gas, so one mole of Cl₂ gas weighs 70.90 g.

Measurement and Quantities

See the Units Appendix for tables of SI fundamental and derived units.

Commonly used units in Chemistry

Quantity	Commonly used units and symbols	Conversions (etc)
amount of substance	mole, mol	1 mol = 6.022×1023 particles
concentration (solutions)	mole per litre, mol L-1 gram per litre, g L-1	(mol L-1)(g mol-1) = g L-1 (g L-1)/(g mol-1) = mol L-1
electric charge	coulomb, C
electric current	ampere, A
electromotive force	volt, V
energy	joule, J kilojoule, kJ	1000 J = 1 kJ
density	gram per litre, g L-1
mass	gram, g kilogram, kg ton	1 tonne = 1000 kg = 1×106 g
molar volume	litre per mole, L mol-1	22.41 L (S.T.P) 22.47 L (25 °C, 101.325 kPa)
pressure	kilopascal, kPa atmosphere, atm millimetre of mercury, mmHg	1 atm = 101.324 kPa = 760.0 mmHg
temperature	kelvin, K degree celsius, °C	TK = TC + 273.15
time	second, s hour, h	1 h = 3600 s
volume	litre, L millilitre, mL cubic centimetre, cm3 cubic decimetre, dm3	1 L = 1000 mL = 1 dm3 = 1000 cm3

Commonly used prefixes

Prefix	Symbol	Factor
tera	T	1012
giga	G	109
mega	M	106
kilo	k	103
milli	m	10-3
micro	µ	10-6
nano	n	10-9
pico	p	10-12