Concentration Calculations

• Titrations are experiments that let you find the volumes needed for 2 solutions to react together completely. If you know the concentration of one of the solutions, you can use the volumes from the titration experiment, along with the reaction equation, to find the concentration of the other solution
• Formula triangle linking concentration, moles and volume

  
• Find the concentration of the acid in mol/dm^3

  1. Work out how many moles of the 'known' substance (NaOH) you have

  2. Use the reaction equation to work out how many moles of the 'unknown' substance (H2SO4) you must have had

  3. Work out the concentration of the 'unknown' stuff
• 0.100 mol/dm^3 x (25.0/1000) dm^3 = 0.00250 moles of NaOH
• 0.00250 moles of NaOH must have reacted with 0.00250 / 2 = 0.00125 moles of H2SO4
• Concentration = number of moles / volume = 0.00125 mol / (30.0 / 1000) dm^3 = 0.041666... mol/dm^3 = 0.0417 mol/dm^3
• All measurements have some uncertainty to them. Titration will often be repeated, and then the average (mean) of these repeated measurements will be calculated. The range of the results can also be found, and can be used to give you an idea of how uncertain the mean value is
• Example

  What's the concentration, in g/dm^3, of the sulphuric solution of concentration 0.0417 mol/dm^3?


  1)Work out the relative formula mass for the acid

  > Mr(H2SO4) = (2 x 1) + 32 + (4 x 16) = 98

  2) Convert the concentration in moles into concentration in grams. So in 1 dm^3

  > Mass in grams = moles x relative formula mass = 0.041666... x 98 = 4.08333... g

  = So the concentration in g/dm^3 = 4.08 g/dm^3