6.7(b)- % of purity

 

Definition:

Purity- only containing that certain element//free from contamination

 

In this page, we will learn how to:

- Carry out calculations involving percentage purity

 

Why find the percentage purity anyway?

Purity is very important. e.g. for analytical standards in laboratories or pharmaceutical products where impurities could have dangerous side effects in a drug or medicine.

 

However in any chemical process it is almost impossible to get 100.00% purity and so samples are always analysed in industry to monitor the quality of the product.

 

 It would not be acceptable in the pharmaceutical industry to manufacture a drug for treating us, with impurities in it, that may have harmful effects.

 

We find the percentage of purity by this formula:

 

         MASS of USEFUL PRODUCT  

% purity=     in TOTAL MASS of SAMPLE   x 100

 

This is how you apply it.

 

sample question:

When dilute hydrochloric acid is added to 5.73g of contaminated calcium carbonate 2.49g of carbon dioxide is obtained.

 

 CaCO3 + 2HCl --> CaCl2 + H20 + CO2

 

Find the percentage purity.

 

If you have viewed our previous page here, the steps on finding the percentage yield is exactly the same as finding the percentage purity.

 

Step 1: 

find the number of mole in 5.73g of CaCO3

Molecular mass of CaCO3

 

  Ca=40  

  C  = 12 

  O = 16  

 

(40x1)+(12x1)+(16x3)= 100

     

 

      Mole  = mass

                  Ar

 

 

 

 5.73 

 100    =  0.0573 mol

 

Therefore:

0.0573CaCO3 + 0.05732HCl --> 0.0573CaCl2 + 0.0573H2O + 0.0573CO2

        |                                                                                       |

     5.73g                                                                                 X

 

To find the mass of X (the total mass of sample), we must take the RMM and multiply it with the mole, 0.573 because:

 

 

 

Mass = mole x Ar

 

 

 

We calculate the RMM the exact same way as calcium carbonate. 

 

CO2:

(12x1)+(16x2)= 44 

 

i.e.  0.0573 x 44 = 2.5212g

 

So now the formula is like this:

 

0.0573CaCO3 + 0.05732HCl --> 0.0573CaCl2 + 0.0573H2O + 0.0573CO2

        |                                                                                            |

     5.73g                                                                                 2.5212g

 

Now we have found the theoretical purity. 

We need to calculate the percentage of purity.

We do this by using the formula from above:

 

                          MASS of USEFUL PRODUCT

% purity   =      in TOTAL MASS of SAMPLE   x 100

 

% purity   =   2.49g

                   2.5212g   x 100

 

% purity  = 98.8 % (3 significant figures)

 

Now since you've learnt all of these, it's time to put your skills to the test!

 

 

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