General Chemistry/Measurement

 General Chemistry Book Cover · Introduction ·  v • d • e

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 NA. 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

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

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