) Why is the Lewis structure for CO2 usually written as O = C = O rather than O ≡ C - O ?

The Lewis structure for CO2 is O = C = O rather than  O ≡ C - O because of the covalent nature of the bonds that makes CO2 a polar molecule and to become stable it is important to fill valence shell of both atoms equally.

CO2 can not be written as O ≡ C - O because then there will be one non-paired electron in the oxygen atom that can disturb the stability of CO2.

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