How to calculate % recovery
Consider a purification procedure, such as sublimation or recrystallization.
This example can also be used in an extraction lab were you know the amount of compound that you started with.
We are using tolueic acid in this example

tolueic acid
C8O2H8    
All masses should be measured on a tared mass balance using weigh paper.  This will help reduce error due to the deviation of the balance.

1.    Record mass of what you started with.

Initial Mass of Tolueic Acid : 62 mg


The initial mass will be the numerator for your percent yield calculation.

This value should be recorded in the Data/Observations of your lab report.

2. Procede with whatever purification/extraction procedure you had planned.

       


This will be the bulk of time spent in the lab.  Unless you had your eyes closed, you should have recorded some observations for the Observation section of your report.

3. Make sure the purified product is dried of any solvents used

    This can be done by:

  • Allowing air to be pulled across the purified compound using the aspirator
  • Press the compound between filter papers until they no longer pick up moisture
  • Set purified compound on a watchglass and air dry




4. Measure mass of dry purified compound

      
    Collected mass: 42 mg  


The collected mass will be the numerator for your percent yield calculation.

This value should be recorded in the Data/Observations of your lab report.

5.  Calculate % recovery

        (Collected Mass /  Initial Mass) x 100%  = % recovery

        (42 mg / 62 mg) x 100% = 68%     




This calculation should be in the conclusions section of your lab report.

6. Does your calculation make sense?

  If your collected mass weighs more than your initial mass your percent recovery will be >100%.  Since we unable to violate any laws of conservation of energy and matter in this lab, your purified compound probably contains some solvent or water. 
Return to step 3 and reweigh.


Remember to include units throughout your calculation.