User blog comment:MiscellaneousSoup/Chat Is Down/@comment-1142365-20130729010635

Balanced equation:

Zn + 2HNO3 -> H2 + Zn(NO3)2

Use the molar gas constant of 24L/mol to find the number of mol of H2:

n(H2) = 8.4/24

From the balanced equation you are given that 1 mol of H2 is equivalent to 1 mol of Zn

Therefore the number of mol of Zn is 8.4/24

Therefore the mass of Zn used is 8.4/24 * Mr(Zn)

= 22.9g (3 significant figures)

The 2nd question is done in the same manner.