There are more atoms of hydrogen in the human body than atoms of any other element but hydrogen contributes
far less than carbon or oxygen to the mass. Chemists are interested in the **number** of atoms present
more than the mass.

Chemists can convert masses of elements into a measure of the number of atoms they contain by using the
**mole**. One mole of any substance is that amount of substance which contains as many particles as there
are atoms of carbon-12 in 12 grams of carbon-12.

Atoms are very small, hence the number of atoms in one mole is very large: there are 6.02 x 10^{23}
atoms in 12g of carbon-12. This number is called the **Avogadro Number, L**. Since the **relative atomic
mass** in grams of all elements contains 6.02 x 10^{23} atoms, 1 mole of atoms of an element equals
the relative atomic mass of the element.

- 1 mole of molecules contains 6.02 x 10
^{23}molecules and equals the**relative molecular mass**. - 1 mole of carbon weighs 12g.
- Therefore 6 g of carbon contains 6/12 moles = 0.5 moles.

Number of Moles = | Mass in Grams |

Mass of 1 Mole |

Moles can be used to work out chemical formulæ. We can find the simplest formula of any compound if we know the amounts of each element present in it.

For example, an oily liquid was found to contain 0.2g of hydrogen, 3.2g of sulphur and 6.4g of oxygen.

Element | Number of Moles | Simplest Ratio |
---|---|---|

Hydrogen | 0.2/1 = 0.2 | 2 |

Sulphur | 3.2/32 = 0.1 | 1 |

Oxygen | 6.4/16 = 0.4 | 4 |

Hence the simplest formula is H_{2}SO_{4}.

The chemical formula can also be determined if the percentage mass of each element is known.

For example methane contains 75% by mass of carbon and 25% by mass of hydrogen.

Element | Moles | Simplest Ratio |
---|---|---|

Carbon | 75/12 = 6.25 | 1 |

Hydrogen | 25/1 = 25 | 4 |

Hence the simplest formula is CH_{4}.

Chemical reactions are often carried out in solution. When you are using a solution, it is important to know how much of the solute is dissolved in a particular volume of solution.

Concentrations are usually measured in grams per cubic decimetre or moles per cubic decimetre. To convert from grams per cubic decimetre to moles per cubic decimetre, you need to know the molar mass of the substance.

For example, the molar mass of sodium hydroxide, NaOH, is 40 g.

A solution containing 80 g dm^{-3} has a concentration of

80 | = 2 mol dm^{-3} |

40 |

Sometimes concentration is referred as **molarity**. A one molar solution (1M) contains one mole in 1
dm^{3} of solution. If you know the concentration and volume of a solution you can easily work out
the number of moles present.

Number of Moles = | M x V |

1000 |

[ M = concentration in mol dm^{-3}, V = volume in cm^{3} ]

Concentrations of solutions can be determined by a technique called **titration**. In this technique, a
solution of concentration to be determined is reacted with a solution of **known** concentration. The
volumes of each solution which exactly react together can be determined often by using an indicator.

Iron is a trace element found in the human body. Iron carries out a vital role in the body: as part of the substance hæmoglobin, present in the blood it is responsible for the transport of oxygen. The amount of iron present in the blood can be accurately determined by titration.

__Example 1__

Determine the concentration of sulphuric acid if 25 cm^{3} of 0.1M sodium hydroxide solution is
neutralised by 20 cm^{3} of sulphuric acid.

25 cm^{3} of 0.1M NaOH contains |
25 x 0.1 | moles | |||

1000 | |||||

= 0.0025 moles |

The equation for the reaction is:

2NaOH (aq) + H_{2}SO_{4} (aq)
Na_{2}SO_{4} (aq) + 2H_{2}O (l)

Hence 2 moles of NaOH reacts with 1 mole of H_{2}SO_{4}

Number of moles of H_{2}SO_{4}
in 20 cm^{3}= |
0.0025 | = 0.00125 | ||

2 |

Number of moles of H_{2}SO_{4}
in 1000 cm^{3}= |
1000 x 0.00125 | = 0.0625 | |||

20 |

Hence concentration is 0.0625 mol dm^{-3}.

__Example 2__

An impure sample of iron of mass 2.55g was dissolved in dilute sulphuric acid and the solution made up to
250 cm^{3}. The solution contained iron (II) ions together with the impurities. 25 cm^{3}
samples of the solution were titrated with potassium manganate (VII) solution of concentration 0.02 mol dm^{-3}.
The average volume required to completely react with the iron (II) ions was 28.5 cm^{-3}. What is the
percentage purity of the iron?

Number of moles of MnO_{4}^{-}
used = |
28.5 x 0.02 | = 5.7 x 10^{-4} | ||

1000 |

The equation for the reaction is:

MnO_{4}^{-} (aq) + 5Fe^{2+} (aq) + 8H^{+} (aq)
Mn^{2+} (aq) + 5Fe^{3+} (aq) + 4H_{2}O (l)

Hence 1 mole of MnO_{4}^{-}reacts with 5 moles of Fe^{2+}. | |||

Number of moles of Fe^{2+} |
= 5 x 5.7 x 10^{-4} | ||

= 2.85 x 10^{-3} | |||

This is contained in 25 cm^{3} of sample. | |||

In 250 cm^{3} there are 10 x 2.85 x 10^{-3} moles of Fe^{2+}. | |||

1 mole of Fe = 56g | |||

Hence mass of iron in 250 cm^{3} |
= 56 x 10 x 2.85 x 10^{-3} | ||

= 1.596 g | |||

Percentage Purity = | 1.596 x 100 | = 62.6 | |

2.55 |

Part of this site was last updated on 21^{st} January 2009.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 England & Wales Licence.

Copyright © Article Gems 2006-09.