Gases & Solutions

• One mole of any gas occupies 24 dm^3 at 20°C
• At the same temperature and pressure, equal numbers of moles of any gas will occupy the same volumes
• At room temperature and pressure (r.t.p.= 20°C and 1 atm) one mole of any gas occupies 24 dm^3. You can use this formula to find the volume of a known mass of any gas at r.t.p.:

  Volume of gas = mass of gas/Mr of gas x 24
• Example

  What's the volume of 319.5 g of chlorine at r.t.p.?

  > Volume = mass/Mr x 24 = 319.5/71 = 108 dm^3
• For reactions between gases, you can use the volume of one gas to find the volume of another

  Example

  How much carbon dioxide is formed when 30 dm^3 of oxygen reacts with carbon monoxide? 2CO(g) + O2(g) -> 2CO2(g)


  > 1 mole of O2 -> 2 moles of CO2 so 1 volume of O2 -> 2 volumes of CO2

  > So 30 dm^3 of O2 -> (2 x 30 dm^3) = 60 dm^3 of CO2
• Lots of reactions in chemistry take place between substances that are dissolved in a solution. The amount of a substance (e.g. the mass or the number of moles) in a certain volume of a solution is called its concentration
• The more solute (the substance that's dissolved) there is in a given volume, the more concentrated the solution
• One way to measure the concentration of a solution is by calculating the mass of a substance in a given volume of solution. The units will be units of mass/units of volume. Here's how to calculate the concentration of a solution in g/dm^3:

  Concentration = mass of solute (g)/volume of solvent (dm^3)
• You can calculate the mass of a solute in solution by rearranging the formula - Concentration = mass of solute (g)/volume of solvent (dm^3) - to mass = conc. x volume
• Concentration can also be given in mol/dm^3:

  Concentration = number of moles of solute (mol) / volume of solvent (dm^3)
• Example

  What's the concentration in g/dm^3 of a solution of sodium chloride where 30 g of sodium chloride is dissolved in 0.2 dm^3 of water?

  > Concentration = 30/0.2 = 150 g/dm^3
• Example

  What's the concentration, in mol/dm^3, of a solution with 2 moles of salt in 500 cm^3?

  1)Convert the volume to dm^3 by dividing by 1000

  > 500 cm^3 / 1000 = 0.5 dm^3

  2)Use the formula

  > Concentration = 2/0.5 = 4 mol/dm^3