S115: Prep Notes

Introduction

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)

Supplies

Short List

  •   800mL of 0.5M HCl (divided between four Florence flasks: 200mL each)
  •   Four 500mL Florence flasks
  •   Magnesium turnings (measured in quantities of 0.30g, 0.61g, 1.22g, 2.43g)
  •   Four balloons (found in cabinet 2 on the bottom shelf in the demo room)
  •   Goggles and gloves

Chemicals

  •   0.5M HCl
  •   Magnesium turnings

Prior to Lecture  (prep time ~ 20 min.)

1.    Prepare 800mL of 0.5M HCl.

       A carboy of 0.5M HCl is already prepared in the demo room.

          Recipe:

                 Add 33mL of 12M HCl to 750mL of distilled water and dilute to 800mL. 

                10L carboy:  add 416mL of HCl / 10L.  Fill carboy about half way and add acid. 

                 Fill carboy up to 10L and shake well.

2.    Divide evenly between the four 500mL Florence flasks (200mL of 0.10 mol each)  or fill to fill line on each flask (flasks in demo box are already marked) from the 10 L carboy described above.

3.     Weigh out the following amounts of magnesium ribbon (usually found in bundled groups of four small vials in demo box):

         1.      0.30g Mg    =   0.0125 mol Mg

         2.     0.61g Mg    =   0.0250 mol Mg        

         3.      1.22g Mg    =   0.0500 mol Mg        

         4.      2.43g Mg    =   0.1000 mol Mg

4.    Pre-stretch the balloons; blow them up to an equal size and let the air out.

5.    Place measured amount of magnesium turnings in each balloon and stretch balloons over the tops of flasks containing HCl.

Safety and Disposal

Handle acids with caution.  Combine chemicals from all flasks to react away leftover magnesium.  Pour contents into Qual drum in lab M124.