The answer is "21.5 L" 21.5 L.
So, start with the balanced chemical equation for the decomposition of hydrochloric acid
2"HCl" -> "H"_2 + "Cl"_22HCl→H2+Cl2
Notice that you have a "2:1"2:1 mole ratio between "HCl"HCl and "Cl"_2Cl2, which means that every 2 moles of the former will produce 1 mole of the latter. The number of moles of "HCl"HCl you have is
"70.0 g" * ("1 mole")/("36.5 g") = 1.9270.0 g⋅1 mole36.5 g=1.92 "moles"moles
As a result, the number of "Cl"_2Cl2 moles is
n_(Cl_2) = n_(HCl)/2 = 1.92/2 = 0.96nCl2=nHCl2=1.922=0.96 "moles"moles
At STP, 1 mole of any ideal gas occupies 22.4 L, which means that the volume of "Cl"_2Cl2 produced will be
n = V/V_("molar") => V = n * V_("molar") = 0.96n=VVmolar⇒V=n⋅Vmolar=0.96 "moles" * 22.4moles⋅22.4 L/("mole") = 21.5Lmole=21.5 "L"L