Chemical calculations with moles

• Theoretical yield


  The mass of a product that you expect to be produced in a chemical reaction
• Even though no atoms are gained or lost during a chemical reaction, it is not always possible to obtain the theoretical yield
• Reasons why theoretical yield may not be obtained

  • Some of the product can be lost when it is separated from the reaction mixture
  • There can be unexpected side reactions between reactants that produce different products
  • The reaction may be reversible
• Calculating theoretical yield


  Using the balanced symbol equation, as shown in Chapter 6 Quantitative chemistry 1
• Percentage yield


  The actual yield as a proportion of the theoretical yield
• Calculating percentage yield


  Actual yield / Theoretical yield x 100
• Atom economy


  The proportion of atoms that you started with that are part of useful products
• High atom economies are more sustainable, as they mean fewer atoms are being wasted in products that are not useful
• Calculating percentage atom economy

  Mass of useful product / Mass of all products x 100
• Concentration


  The number of moles of solute per volume of solution (mol/dm³)
• Greater mass of solute in solution


  Greater number of moles of solute, and therefore greater concentration
• Same number of moles of solute dissolved in a smaller volume

  Greater concentration
• Titration


  An experimental technique to work out the concentration of a solution
• Use a beaker and funnel for titration
• Moles of gases


  At room temperature and pressure, the same number of moles of a gas will occupy the same volume (24 dm³)
• Calculating moles of a gas


  Volume of gas / 24 dm³
• Calculating volume of a gas

  Moles of gas x 24 dm³
• Key terms


  • atom economy
  • burette
  • room temperature and pressure
  • concordant
  • theoretical yield
  • end point
  • percentage yield
  • pipette
  • titration
  • titre
  • useful yield
• Concentration


  The amount of solute dissolved in a given volume of solution
• If the same number moles of solute is dissolved in a smaller volume of solution

  The concentration will be greater
• Titration


  An experimental technique to work out the concentration of an unknown solution in the reaction between an acid and an alkali
• Titration procedure


  1. Use a pipette to extract a known volume of the solution with an unknown concentration

  2. Add the solution of unknown concentration to a conical flask and put the conical flask on a white tile

  3. Add a few drops of a suitable indicator to the conical flask

  4. Add the other solution with a known concentration to the burette

  5. Carry out a rough titration to find out approximately what volume of solution in the burette needs to be added to the solution in the conical flask

  6. Add the solution from the burette to the solution in the conical flask 1 cm³ at a time until the end point is reached

  7. The end point is when the indicator just changes colour

  8. Record the volume of the end point as your rough value

  9. Repeat steps 1-7, but as you approach the end point add the solution from the burette drop-by-drop and swirl the conical flask in between drops

  10. Record the volume of the end point
• Burette


  • Use a beaker and funnel to fill
  • Read the volume from the top of the meniscus
  • Use one hand to control the flow rate
• Conical flask


  • Swirl with other hand whilst drops are being added
• Titration procedure


  1. Record the volume of the end point

  2. Repeat the titration until you get concordant titres

  3. Calculate the mean of the concordant results
• Titre


  The volume of solution that you have added from your burette
• Concordant


  Titres are within 0.1 cm³ of each other
• Calculating concentration


  1. Write a balanced symbol equation for the reaction

  2. Calculate the moles used from the known solution

  3. Use the ratio from the balanced symbol equation to deduce the moles in the unknown solution

  4. Calculate the concentration of the unknown solution
• To calculate the moles used from the known solution: moles = concentration (mol/dm³) x volume (dm³)
• To calculate the concentration of the unknown solution: concentration (mol/dm³) = moles / volume (dm³)
• Experiment titration


  1. Use a pipette to extract a known volume of the solution with an unknown concentration

  2. Add the solution of unknown concentration to a conical flask and put the conical flask on a white tile

  3. Add a few drops of a suitable indicator to the conical flask

  4. Add the other solution with a known concentration to the burette

  5. Carry out a rough titration to find out approximately what volume of solution in the burette needs to be added to the solution in the conical flask. Add the solution from the burette to the solution in the conical flask 1cm3 at a time until the end point is reached

  6. The end point is when the indicator just changes colour

  7. Record the volume of the end point as your rough value

  8. Repeat steps 1-7, but as approach the end point add the solution from the burette drop-by-drop. Swirl the conical flask in between drops

  9. Record the volume of the end point
• Pipette


  Measures a fixed volume only
• End point


  When the indicator just changes colour
• what is the yield of a reaction?

  mass of product obtained from the reaction
• What is the theoretical yield of a reaction?

  maximum mass of the product that could have been produced
• Why is the actual yield always less than the theoretical yield?

  reaction may be reversible

  some of the product can be lost on separation

  unexpected side reactions between reactants
• What is the percentage yield?

  actual yield as a proportion of theoretical yield
• How is percentage yield calculated?

  actual yield/theoretical yield x 100
• What is atom economy?

  measure of how many atoms of the reactants end up as useful products
• Why is a high atom economy desirable?

  results in less waste/is more sustainable