Question

H2 (g) +Cl2 (g) -> 2HCl (g)
how many molecules of HCl are produced from 100 g Cl2 ?

Answer 1

mols Cl2 = grams/molar mass = 1.00/71 = ?
mols HCl produced = mols Cl2 x 2 (from the coefficients)
Then you know mols HCl x 6.022E23 molecules/mol = ?