[Ch3m]

I have some questions regarding introductory chem. I appreciate any *delete me* Thanks in adv.! =]

-What exaclty is the atomic mass? Can it be measured in both amu/atom and g/mol?
For example: Lets take Titanium (atmoic mass of 204.4) Does the atomic mass mean that there are 204.4 amu per Ti atom and 204.4 g/ mol of Ti?

-Are molecules and atoms used interchangeably? Also, is a mole unit less? You can substitute any unit for Avogadro's number right? For example: 6.022 x 10^23 atoms or 6.022 x 10^23 molecules or moles, etc.

Given the molecular formula: 12 CH₄, is the coefficient (12 in this case) always the number of moles of the element? Also, is the subscript 4 always the number of atoms or molecules?

Thanks!

[Yggdrasil]

I have some questions regarding introductory chem. I appreciate any *delete me* Thanks in adv.! =]

-What exaclty is the atomic mass? Can it be measured in both amu/atom and g/mol?
For example: Lets take Titanium (atmoic mass of 204.4) Does the atomic mass mean that there are 204.4 amu per Ti atom and 204.4 g/ mol of Ti?

The atomic mass represents both.  That's why the atomic mass isn't given in any units on the periodic table.  You can take it to mean amu/atom or g/mol depending on whether you're considering single atoms or a large collection of atoms.

-Are molecules and atoms used interchangeably? Also, is a mole unit less? You can substitute any unit for Avogadro's number right? For example: 6.022 x 10^23 atoms or 6.022 x 10^23 molecules or moles, etc.

An atom is a fundamental chemical unit that cannot be split into anything more fundamental by chemical means.  A molecule consists of two or more atoms that are bonded together by chemical means.  So, for example, an oxygen atom is O but an oxygen molecule is O₂.

Avogadro's number is a unitless quantity, kind of like a dozen.  Just as you can have a dozen eggs, a half-dozen cookies, or two dozen apples, moles can refer to atoms, molecules or anything else you can think of.  You can even have a mole of moles

Given the molecular formula: 12 CH₄, is the coefficient (12 in this case) always the number of moles of the element? Also, is the subscript 4 always the number of atoms or molecules?

In a chemical formula, the subscripts tell you the number of each type of atom in a molecule.  For methane (CH₄), the subscripts tell you that methane consists of one atom of carbon bonded to four atoms of hydrogen.  Similarly, the formula for something like table sugar (C₁₂H₂₂O₁₁) tells you that there are 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms in a molecule of table sugar.

In a chemical reaction the coefficients can mean various things.  Usually, one would take them to mean number of atoms or molecules.  So in the combustion of methane:

CH₄ + 2O₂ --> CO₂ + 2H₂O

One molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.  But, you can multiply this chemical reaction by any whole number value and still come up with something that makes sense.  For example,

2CH₄ + 4O₂ --> 2CO₂ + 4H₂O

is still a balanced chemical reaction.  Now, consider this balanced chemical reaction:

6.02x10²³CH₄ + 1.204x10²⁴O₂ --> 6.02x10²³CO₂ + 1.204x10²⁴H₂O

It's still balanced, so the equation is still valid.  And in terms of moles, this becomes:

(1 mole)CH₄ + (2 moles)O₂ --> (1 mole)CO₂ + (2 moles)H₂O

So, you can interpret coefficients to represent either molecules/atoms or moles.

(NB: to me part of the beauty of chemisty is that it allows you to connect what happens at the microscopic level [i.e. in terms of atoms and molecules] to what happens on the macroscopic level [i.e.  in terms of things we can see], and vice versa.  So, while we might not be able to see the combustion of one molecule of methane, we can see the combustion of one mole of methane and use that information to see what happens at the molecular level).

[Ch3m]

Wow! Thanks a lot for your help. I'm almost done with stiochiometry right now but I'm wondering if things will get considerably harder as I go further into the textbook. What would you reccomend that I have a good grasp at so that it will make my chem studies more easy? I realize that you need to have good nomeclature skills to transform words into molecular formulas but is there anything else? Do I need to memorize the polyatmoic ions? There's about 24 of them that my book has.

Also, in your opinion, what would you think is the hardest seciton in Chemistry? 

Thank you very much! =]

[Yggdrasil]

Stoichiometry is definitely something you should have a good grasp of in your chemistry course as a lot of chemistry builds off of a good knowledge of stoichiometry.  Nomenclature at the high school level is important, but not too hard.  The main focus should be knowing how to differentiate between nomenclature for covalent/molecular compounds and ionic compounds.

I would advise against straight memorization of a whole page of polyatomic ions unless your teacher tells you to.  Yes, there are some that you will need to have memorized, but I find that it might be better to memorize them through familiarity (seeing them in so many practice problems that they get committed to memory).  If in doubt, I'd suggest asking your teacher about this.

As for the hardest section of chemistry, it really depends on how you learn and what types of learning is hardest for you.  Someone who is good at math and working with numbers might not find stoichiometry and chemical equilibrium difficult, but these topics may be hard for someone who can't work out math problems.  Similarly someone who grasps concepts easily may find ideal gases and kinetic theory enjoyable to learn, but they could be hard if you can't grasp the concepts and understand what's happening at the molecular level.  If concepts are hard for you to grasp, ideal gases and kinetic theory may be difficult.

[Bakegaku]

I probably wouldn't try to plainly memorize the names of polyatomic ions, but try to recognise patterns among them.  I.E. most of them end in "-ite" or "-ate."  Also take note that if two ions have the same central atom, the "-ate" will have more oxygen's than the "-ite" (Nitrate is NO₃^- while Nitrite is NO₂^-).  Sometimes they also have "Hypo" (less than) and "Hyper" (or just Per)(greater than), depending on the number of oxygens in the ion.  (Hypochlorite: ClO^-, Chlorite: ClO₂^-, Chlorate: ClO₃^-, Perchlorate: ClO₄^-)

Other than that, some others that don't follow the rules so well that you may need to are acetate, peroxide, superoxide, cyanide, hydroxide and ammonium. 

I hope that rambling doesn't confuse you.  Other than that I think I'd say you should make sure you know well how bonding works (basic electron configurations, that numbers and types of bonds different elements make, etc.)