Isoelectronic means having the same electron configuration (i.e. the same number of electrons).  and  both have the same number of electrons (18), while sulfur has 16, phosphorous has 15, neon has 10, and the sodium cation has 10.

When drawing a Lewis dot structure, we are always trying to reach an electron count where all atoms involved are stable and (usually) have full octets. We are also trying to estabilsh a structure in which we have the smallest formal charge possible. The general rule is first to draw out all of the elements involved and their valence electrons, then start piecing them together trying to reduce the formal charge and get all elements involved to an octet. There are a couple exceptions to the octet rule.  

Sodium and chlorine form an ionic bond, meaning that one atom will donate an electron and the other will receive it. This gives each atom a charge. Chlorine has seven valence electrons, while sodium has one valence electron. For each atom to arrive at an octet, sodium will need to lose one electron and chlorine will need to gain one electron. This would give chlorine a negative charge, and sodium a positive charge.

Thus, the answer is a sodium with a positive charge (due to one lost electron) and a chlorine with eight electrons and a negative charge (due to one electron gained). 

When drawing a Lewis dot structure, we are always trying to reach an electron count where all atoms involved are stable and (usually) have full octets. We are also trying to estabilsh a structure in which we have the smallest formal charge possible. The general rule is first to draw out all of the elements involved and their valence electrons, then start piecing them together trying to reduce the formal charge and get all elements involved to an octet. There are a couple exceptions to the octet rule.

In this case, boron actually has an incomplete octet. Though there are resonance forms in which boron has a full octet, when you calculate the formal charge of these configurations it will not be zero.

Intrinsic properties do not depend on the amount of a substance present. Mass, heat, and enthalpy are all dependent on the amount of substance.

The law of constant composition can't be applied to solutions because it only applies to pure samples. Solutions are not pure samples, since they contain more than just 1 compound.

This is the very definition of an intermediate. It is formed in one step within the mechanism of the reaction, but then something else happens to it as the reaction continues to completion; thus, it does not show up in the overall reaction. 

Knowing there is at least one hydrogen atom and one oxygen atom in this compound means the total molar mass must be at least . If we first assume  is equivalent to only 1 mole of hydrogen, we get a total molar mass of . This number is too low to make logical sense. 

Therefore, we must next assume there are 2 moles of hydrogen. Using this assumption, we can find the hypothetical molecular mass.

Use this molecular mass to calculate the mass of oxygen.

Therefore, there must be 2 moles of hydrogen and 2 moles of oxygen, making the molecular formula .

This compound is called peroxide.

Use the molar mass and percent by mass of carbon to find the moles of carbon in the compound.

We know that the compound is made solely of carbon and hydrogen. This allows us to find the percent by mass of hydrogen.

Use the molar mass and the percent by mass of hydrogen to find moles of hydrogen in the compound.

The molecular formula is .

Because 7 and 16 have no common factors, the compound cannot be further reduced (in a numerical, not chemical sense).  is also the empirical formula.

Use the molar mass and percent by mass of sulfur to find the moles of sulfur in the compound.

Since the compound consists of only sulfur and element X, we can subtract to find the percent by mass of element X.

Use the percent by mass to find the mass per mole of compound of element X.

Because we are told there are one-half as many moles of X as there are of sulfur, we know there are 2 moles of X (as compared to the 4 moles of sulfur). Divide the mass of element X by the moles of element X to find its atomic mass and its identity.

By consulting a periodic table, we can find that element X is silicon.

Therefore, the molecular formula is  and the empirical formula is .

Use the molar mass and percent by mass of sodium to find the moles of sodium in the compound.

Since the compound consists of only sodium and element X, we can subtract to find the percent by mass of element X.

Use the percent by mass to find the mass per mole of compound of element X.

Because there is a 1:1 ratio, between sodium and element X, they must have an equal number of moles in the compound. Since there are 2 moles of sodium, there must also be 2 moles of element X. Divide the mass of element X by the moles of element X to find its atomic mass and its identity.

By consulting a periodic table, we can find that element X is oxygen.

Therefore element X is Oxygen.

Therefore, the molecular formula is  and the empirical formula is .