Which compound has the highest melting point?
silicon carbide (sic), a network solid
octane (c8h18), a molecular compound
carbon dioxide (co2), a common gas
water (h2o)

silicon carbide (SiC), a network solid

Explanation:

SiC has a giant network of covalent structure which takes a lot of energy to break than just a single covalent bond. In the network there are many Si and C atoms. All the atoms in the network are linked to each other by strong covalent bonds. SiC forms a lattice structure, in which both the Si and C atoms are sp³ hybridized. In the SiC crystal lattice the smallest unit is a tetrahedron of a central Si atom surrounded by four C atoms or a central C atom surrounded by four Si atoms.

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