Polyacrylonitrile is a polymer made from the monomer acrylonitrile, CH2CHCN. Polyacrylonitrile is used in making Acrilan™ fibers
used widely in rugs and upholstery fabric. If ignited,
this fiber can release a poisonous gas. In the case of
a fire, what danger might rugs and upholstery made
of this polymer present?

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2