C3H8(g) + 4 Cl2(g) → C3H4Cl4(g) + 4 HCl(g)
A 6.0 mol sample of C3H8(g) and a 20. mol sample of Cl2(g) are placed in a previously evacuated vessel, where they react according to the equation above. After one of the reactants has been totally consumed, how many moles of HCl(g) have been produced?