Home > Chemistry > Calculations (Unit 1) > The mole
Chemistry
The mole
One mole of a substance is its gram formula mass (GFM).
One mole of a substance contains 6.02 x 10^{23} formula units.
This number is known as the Avogadro constant.
The Avogadro constant can be found in the SQA data booklet, and is expressed as 6.02 x 10^{23} mol^{-1}.
The term formula unit is a general term that relates to the type of particles that make up a substance. In general, it refers to the formula normally used for the substance.
Equimolar quantities of substances contain equal numbers of formula units.
The idea of the mole links the mass of a substance to the number of formula units it contains.
The calculations below involve calculating number of formula units, given number of moles, and vice versa.
The link here is:
1 mole ↔ 6.02 × 10^{23} formula units
How many molecules are contained in 0.65 mole of sulphur dioxide, SO_{2}?
Find: m molecules from 0.65 mole of SO_{2}
Link: 6.02 × 10^{23} formula units ↔ 1 mole
Convert: 6.02 × 10^{23} molecules ↔ 1 mole
Proportion: There are 6.02 × 10^{23} molecules in 1 mole. Hence, in 0.65 mole:
m = 0.65 × (6.02 × 10^{23})
m = 3.91 × 10^{23}
Answer: 3.91 x 10^{23} molecules are in 0.65 mole of SO_{2}.
How many moles of hydrogen contain 5.5 x 10^{24} molecules of H_{2}?
Find: m moles from 5.5 x 10^{24} molecules
Link: 1 mole ↔ 6.02 × 10^{23} formula units
Convert: 1 mole ↔ 6.02 × 10^{23} molecules
Proportion: There is 1 mole for every 6.02 × 10^{23} molecules. Hence, for 5.5 x 10^{24} molecules:
m = (5.5 x 10^{24}) ÷ (6.02 × 10^{23})
m = 9.14
Answer: 9.14 moles contain 5.5 x 10^{24} molecules of H_{2}.
The examples involve calculating number of formula units from a given mass, and vice versa.
The link here is:
mass of 1 mole ↔ 6.02 × 10^{23} formula units
As the mass of moles is involved, it is necessary to calculate the GFM of the substance.
How many molecules are in 150 g of water, H_{2}O?
GFM: 2 x H and 1 x O
2 × 1.0 = 2.0
1 × 16.0 = 16.0
GFM = 18.0 g
Find: m molecules from 150 g
Link: 6.02 × 10^{23} formula units ↔ 1 mole
Convert: 6.02 × 10^{23} molecules ↔ 150 g
Proportion: There are 6.02 × 10^{23} molecules in 150 g. Hence, in 150 g:
m = (150 × 6.02 × 10^{23}) ÷ 18
m = 5.02 × 10^{24}
Answer: 5.02 x 10^{24} molecules are in 150 g of water, H_{2}O.
To work out the number of atoms, multiply the number of molecules by three, which is the number of atoms in one molecule.
What mass of carbon disulphide (CS_{2}) contains 2.04 x 10^{22} molecules of CS_{2}?
GFM: 1 x C and 2 x S
1 × 12.0 = 12.0
2 × 32.1 = 64.2
GFM = 76.2g
Find: m grams from 2.04 x 10^{22} molecules
Link: 1 mole ↔ 6.02 × 10^{23} formula units
Convert: 76.2 g ↔ 6.02 × 10^{23} molecules
Proportion: There are 6.02 × 10^{23} molecules in 76.2 g. Hence, for 2.04 x 10^{22} molecules:
m = (76.2 × 2.04 x 10^{22}) ÷ (6.02 × 10^{23})
m = 2.58
Answer: 2.58 g contains 2.04 x 10^{22} molecules of CS_{2}.
What mass of sodium chloride (NaCl) would contain 9.03 x 10^{21} ions?
GFM: 1 x Na and 1 x Cl
1 × 23.0 = 23.0
1 × 35.5 = 35.5
GFM = 58.5g
Note that one formula unit of NaCl contains two ions.
Find: m grams from 9.03 x 10^{21} ions
Link: 1 mole ↔ 6.02 × 10^{23} formula units
Convert: 58.5 g ↔ 2 × (6.02 × 10^{23}) ions
Proportion: 2 × (6.02 × 10^{23}) ions are in 58.5 g. Hence, 9.03 x 10^{21} ions are in:
m = (58.5 × 9.03 x 10^{21}) ÷ (2 × (6.02 × 10^{23}))
m = 0.44
Answer: 0.44 g of NaCl contains 9.03 x 10^{21} ions.
BBC © 2014 The BBC is not responsible for the content of external sites. Read more.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.