Magnesium metal is dissolved in HCl in 500mL Florence flasks covered with balloons.  The moles of each reagent are changed in each flask in order to demonstrate the limiting reagent concept.  In flasks 1 and 2, a small amount of Mg is used and therefore the metal is the limiting reagent.  In flask 3, the reagents are added in a stoichiometric ratio.  In flask 4, excess Mg is added and HCl becomes the limiting reagent.  The hydrogen gas evolved is collected in the balloons, and the size of each balloon is proportional to the amount of hydrogen produced.

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) 

Non-inflated balloons over flaskInflated balloons over flask

To Conduct Demonstration

  1. Lift the balloons one at a time so that the Mg falls into the HCl in each flask.  Swirl to speed up reaction.  Make sure all the Mg is emptied out of the balloon.

  2. Each flask contains 0.1 mol of HCl.  Magnesium is present in the following amounts:

      Flask 1         0.0125 mol Mg         0.1 mol HCl         excess HCl

      Flask 2         0.0250 mol Mg         0.1 mol HCl         excess HCl

      Flask 3         0.0500 mol Mg         0.1 mol HCl         stoichiometric HCl/Mg ratio

      Flask 4         0.1000 mol Mg         0.1 mol HCl         excess Mg

  1. Flask 1 and 2 are limited by smaller quantities of Mg.  Flask 3 will react to use both reagents evenly and completely.  Flask 4 will produce only the same amount of hydrogen as Flask 3 and have excess Mg left over, since the reaction is limited by the HCl.  Flask 4 will produce H2 faster than flask 3, but both balloons in the end will be nearly the same size.

Reaction Time: Perform demo at the beginning of lecture and leave for the rest of the class  period to develop.


Handle acids with caution.  


Mary DuBois, Spring 1987
Lora Ruffin and Michael Polk, Summer 2009