From the equation you get that you need 2 moles of HCl to dissolve one
mol of Fe. And as a result you will get one mol of FeCl2 and one mol
According to the Periodic Table of Elements one mol of Fe weighs
55.85g, for the purposes of this problem you can round it to 56g.
You have 2.8g of Fe, that is: 2.8/56 = 0.05 moles of Fe.
Now you know that for each mol of Fe you need two of HCl, then for
0.05 moles of Fe you will need 0.05*2 = 0.1 moles of HCl.
One mol of HCl weighs 36.5g aprox.; then 0.1 moles will weigh 3.65g.
This is the amount of HCl in grams that you need dissolve a 2.8g iron
Recall that for each mol of Fe dissolved you will get one mol of H2,
since 0.05 moles of Fe are dissolved, then 0.05 moles of H2 are
produced as a result of the reaction.
Since one mol of H2 weighs 2g, 0.05 moles of H2 will weigh 0.1g, and
this is the amount of H2 produced by the complete reaction on the iron
For references about molecular weight and mol weights see:
"Molecular mass - Wikipedia, the free encyclopedia":
This page will be also useful for understanding this kind of problems
and many related:
"Reaction Equations" from the Faculty of Science Website of the
University of Waterloo, Ontario, Canada:
For a nice printable Periodic Table:
"Detailed Color Printable Periodic Table":
molecular weight mol
reactions moles weight
I hope this helps you. Feel free to request for a clarification if you need it.