# 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 substance 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