in todays experiment what length of magnesium ribbon would be required to generate 42.00 ml's of H2?

Answer 1

determine the mass...
Mg+2HCl>>>MgCl2 + H2
at stp (your conditions not known), 42ml H2 equivalent to .042/22.4 moles, or .042*23/22.4 grams Mg.

Length=massMg/(mass/length)

sometimes on Mg ribbon, the mass /length is labeled on the container, but you can determine it by massing measuring a known length.