4Al(s) + 3O2(g) --> 2Al2O3(s) This is the balanced.
From the equation:
4 moles of Al required 3 moles of O2 to produce 2 moles of Al2O3
3 moles of O2 reacted with 4 moles of Al to produce 2 moles of Al2O3
1 mole of O2 reacted with 4/3 moles of Al to produce 2/3 moles of Al2O3 (Divide by 3)
4.5 moles of O2 reacted with (4/3 *4.5) moles of Al to produce (2/3*4.5) moles of Al2O3
4.5 moles of O2 reacted with 6moles of Al to produce 3moles of Al2O3
(3) is the answer. 6 mol of Al.
Answer :
Lewis-dot structure : It shows the bonding between the atoms of a molecule and the unpaired electrons present in the molecule.
The given molecule is, 
Beryllium has '2' valence electrons and fluorine has '7' valence electrons. Beryllium is the central atom and fluorine is the terminal atom.
Total number valence electrons in
= 2 + 2(7) = 16
According to the Lewis-dot structure, there are '4' number of bonding electrons and 12 number of non-bonding electrons (lone-pair).
The Lewis-dot structure is shown below.
The answer is (2) 10.0 mol. The equation given is balanced. So the ratio of mol number of compound is the ratio of the number before the compound. The HCl and CO2 ratio is 2:1. So the answer is 10.0 mol.
Answer: A. -396 kJ
Explanation:
The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.

Reversing the reaction, changes the sign of 

On multiplying the reaction by 2, enthalpy gets multiplied by 2:

Thus the enthalpy change for the reaction
is -396 kJ.
Mass number is apart of (what ever subject it is)