Laboratory Investigation:

Magnesium Burning 

 

Introduction:

This experiment introduces stoichiometry, the science of finding out how much stuff is necessary in a chemical equation. 

 

Method:

  • Prepare a strip of magnesium about 10 cm long.  This is about a seventh of a gram ( ~ 0.15 grams).

  • Place the strip of magnesium in a crucible and weigh it.  Be sure to determine the mass of the cricible alone as well so that its mass can be subtracted later. 

  • Set up the cricible on a tripod above a Bunsen burner and begin heating. Be careful not to look too closely at the burning magnesium, it's light is bright enough to be dangerous for the eyes.

 

  • After the magnesium has finished burning, turn off the Bunsen burner and let the material cool for several minutes. 

 

Results:

  • Observe the changes in the magnesium:

 

  • One observation which needs to be made is with the balance to see any changes in the mass.

 

 

 

Conclusion:

  • There has obviously been a chemical change because several chemical properties of the magnesium have been modified:  the color, the texture and the mass.

  • The increase in mass is due to the fact that oxygen from the air has combined with the magnesium to make magnesium oxide, MgO.

  • The chemical equation, Mg + O2 MgO shows this reaction but it needs to be balanced to make 2Mg + O2 2MgO. 

  • Using stoichiometry, we can convert this eqation into an equation with moles: 
     2 mol Mg + 1 mol O2 2 mol MgO. 

  • Next, we convert to grams using atomic masses obtained from the periodic table:
     48g Mg + 32g O2 80g MgO

  • Lastly, we determine the same thing in the proportions we used.  In other words, we used only 0.15g of Mg (not 48g) so everything needs to be divided by 320.  So 80 / 320 = 0.25 g.  If we burn 0.15 g of Mg, we obtain 0.25 g of MgO.