Chemistry only- higher tier
The table below gives the formula and mass of 1 mole for a number of gases.
|gas||molecular formula||Ar or Ar||mass of 1 mole/g|
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 200C or 293K 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 200C and 1 atmosphere pressure 1 mole of any gas will occupy 24dm3 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. It 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 dm3 at 200C and 1 atmosphere pressure.
|gas||molecular formula||Ar or Ar||mass of 1 mole/g||volume gas will occupy at RTP/dm3|
Example. Calcium carbonate (CaCO3) decomposes when heated according to the equation below:
The Haber process for making ammonia gas can be shown by the equation below:
Here 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 500cm3 of nitrogen 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: