I am working on my summer assignment for AP chemistry. Please help!?

Here are some questions I have from the packet. Please help me out. If you do not know, please do not try to help because I would like to get a good grade on the assignment, and if your answer is wrong, I won't understand the problem. Also, if you can, please explain your answers if an explanation seems important because I don't want to cheat...I want to learn. Thanks a bunch!
1. What is the difference between a molecular, condensed, and structural formula?

2. Is the central atom in a pollyatomic ion always a nonmetal? Explain.

3. Why is the formula of methan e written CH4, and phosphine written PH3, whereas the formula of hydrogen sulfide is written H2S and hydrogen chloride written HCl? (i.e. why does the hydrogen follow the C or P, but precede the S or Cl?)

4. The condensed formula for acetic acid is CH3CO2H. What is its empirical formula; what is its molecular formula; what is its structural formula?

5. How many mol of N atoms are there in 6 mol of CH3C6H2(NO2)3?

Answer:
1) A molecular formula is the simplest formula which give the ratios of the numbers of atoms in a molecule. For example, C2H6 is the molecular formula for the compound Ethane, because it contains 2 Carbon atoms and 6 Hydrogen atoms in each molecule. A structural formula depicts not only the number of atoms, but the spacial arrangement of those atoms as well. See the first site below for a structural formula of Ethane. An example of a condensed formula would be by writing Ethane as CH3-CH3 showing that each Carbon is connected to the other Carbon atom and 3 Hydrogens as well.
2) Usually, but not always. For the polyatomic ions Ammonia, NH4+; Sulfate, SO4-2; Nitrate, NO3-, and Phosphate, PO4-3, the central atom is a nonmetal. A common example of a metal as the central atom is Ferricyanide (or hexa-cyanoferrate) which is Fe(CN)6 with a negative 3 charge.
3) This order is just convention. The least electronegative element is usually written first and then the more electronegative element. When one of the elements is Hydrogen, it is usually written last in covalently bonded compounds and written first in more ionicly bonded compounds (especially acids).
4) The empirical formula is C2H4O2. The molecular formula is often written as CH3COOH. See the third site below for a structual formula.
5) Every molecule has 3 Nitrogens. 6 moles would have 6*3 = 18 moles of Nitrogen.

#5: 6x3 =18 moles of N atoms

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I am working on my summer assignment for AP chemistry. Please help!?

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