[war485]

Hello.  My group and I did an experiment weeks ago that involved 0.75 grams of magnesium reacting with 20 mL of 1.0M HCl.  There was an excess of magnesium metal in the beaker.

It resulted in a change of pH from 1 to 9.1 and the mass of the system decreased by 0.643 grams.  By balancing the 3 equations and using molar masses, We're still missing like 0.5 g of the total mass loss due to the emission of gases (no lid on top of the beaker when it reacted).

My three chemical equations were:

Mg _(s) + 2HCl _(aq) --> MgCl_{2 (aq)} + H_{2 (g)}
MgO _(s) + H₂O --> Mg(OH)_{2 (aq)}
Mg _(s) + 2H₂O _(l) --> Mg(OH)_{2 (s)} + H_{2 (g)}

Keeping in mind that I only reacted HCl with Mg.  What am I missing?  There was also a funny smell during the reaction.  It can't be hydrogen since that is not odorous, I think...  What could it be?  Any hints/solutions/logical ideas will be very helpful for my group.  Thanks a ton!   Justin

[Equi]

I can definitely tell you that elemental Magnesium or MgO won't react solely with water, or at least not to a significant extend. The loss of mass is due to the production of volatile hydrogen gas.

By the way: such reactions were formerly used for hydrogen production (Kipp's apparatus).

[Dan]

There was also a funny smell during the reaction.  It can't be hydrogen since that is not odorous, I think...  What could it be?

The funny smell is due to your school's chemistry budget. The magnesium was probably not very high grade, and the smell is due to side reactions of impurities. For the purposes of your calculations, you can ignore this.

[war485]

Hmm.... cheap budget materials and magnesium metal.  That makes sense for the smell then, which smelled something like sulphur (it makes me feel like throwing up...).  It makes more sense this way, but the impurities in the magnesium sorta ruins the whole calculation thing for hydrogen gas...  I'll let the others know now.  Thanks guys!