Chemistry only- higher tier

The table below gives the formula and the mass of 1 mole for a number of different gases.

gas | molecular formula | A_{r} |
mass of 1 mole/g |
---|---|---|---|

hydrogen | H_{2} |
2 | 2 |

helium | He | 4 | 4 |

oxygen | O_{2} |
32 | 32 |

methane | CH_{4} |
16 | 16 |

carbon dioxide | CO_{2} |
44 | 44 |

butane | C_{4}H_{10} |
58 | 58 |

Remember gases are mostly empty space with
large gaps between the particle. The volume that
a gas occupies depends on its temperature
and pressure. If you heat a gas up it will expand
and take up more space and if you cool it then its volume will decrease.
Squashing a gas or
putting it under pressure will also decrease its
volume while reducing the pressure on a gas
will cause it to expand. So when you talk about the volume a
gas occupies you should really state
the temperature and pressure as well.

In gcse chemistry we are mostly dealing with gases at room
temperature and pressure (RTP), that is 20^{0}C or 293Kelvin
and 1 atmosphere pressure.
According to Avogadro's theorem 1 mole of any gas will occupy the same volume at any given pressure
and temperature. This means for example that 2g of hydrogen gas, that is 1 mole
of hydrogen will occupy the
same volume as 32g of
oxygen (1 mole of oxygen) or 44g of carbon dioxide (1 mole
of carbon dioxide) or 58g of butane 9 (1 mole of butane)
or 4g of helium (1 mole of helium). At 20^{0}C and 1
atmosphere pressure
1 mole of any gas will occupy 24dm^{3} or 24 litres. This is
called the molar volume of a gas.

The table below is almost identical to the one above with the exception of the last column which shows the
molar volume, that is the
volume occupied by 1 mole of that particular
gas. As you can see 1 mole of any
gas occupies 24 litres or 25 dm^{3}
at 20^{0}C and 1 atmosphere pressure.

gas | molecular formula | A_{r} |
mass of 1 mole/g | volume gas will occupy at RTP/dm^{3} |
---|---|---|---|---|

hydrogen | H_{2} |
2 | 2 | 24 |

helium | He | 4 | 4 | 24 |

oxygen | O_{2} |
32 | 32 | 24 |

methane | CH_{4} |
16 | 16 | 24 |

carbon dioxide | CO_{2} |
44 | 44 | 24 |

butane | C_{4}H_{10} |
58 | 58 | 24 |

Calcium carbonate (CaCO_{3}) decomposes when heated according to the equation below:

A

Step 1- You need a balanced symbolic equation. Now the equation above is balanced and from it you can see that 1 mole of calcium carbonate produces 1 mole of carbon dioxide gas.

Step 2- calculate the number of moles of calcium carbonate present:

Use the formula:

So the number of moles calcium carbonate present = 50g/100 = 0.5 moles.

Now from the balanced symbolic equation above we already know that:

1 mole of CaCO_{3}will produce 1 mole of carbon dioxide (44g) gas.

So 0.5 moles of CaCO_{3} will produce 0.5 moles of carbon dioxide (22g) gas.

So 0.5 moles of CO_{2} gas will occupy 24dm^{3} x 0.5 = 12dm^{3}

The formula to use to calculate the volume of a gas
relies on knowing how many moles of gas you have,
you simply multiply the number of moles by the molar volume,
that is 24dm^{3}. This of course assumes that the temperature and
pressure are at room temperature and pressure.

The Haber process is used to make ammonia gas (NH_{3}), this is shown in the equation below:

This balanced symbolic equation shows that 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to produce
2 moles of ammonia gas.

What volume of
ammonia gas will be produced if 500cm^{3} of nitrogen gas react with an excess of hydrogen gas at RTP?

This time instead of using reacting masses the question gives volumes of reactants. However we know that the
temperature
and pressure stay constant. We also know that:

- 1 mole of nitrogen produces 2 moles of ammonia. or
- 24dm
^{3}of nitrogen will produce 48dm^{3}of ammonia.
so
- 5oocm
^{3}of nitrogen will produce 1000cm^{3}of ammonia.

Alternatively you can work out the number of moles of nitrogen gas present by simply dividing the reacting volume of the nitrogen gas; that is 500cm^{3} by 24 000 cm^{3}, that is the reacting volume of nitrogen divided by the molar volume, which gives 0.02 moles of nitrogen gas reacting.

Now simply use the formula shown above to calculate the volume of ammonia produced:

This will give 0.02moles x 24dm