Need chemistry refresher (stoichemetry?)

[Mothergoose729]

I need to take the entrance exam for my chem 400 class soon. I took chemistry in high school, I would rather skip the introduction to chem class as it is not required for my major and would set me back a number of units. I did will in my chemistry class but it was several years ago. I am pretty sure I remember the basic ideas of electron orbits, positive and negative charges, how the order and mass of elements is generally determined, ect. I will be enrolled in caculus the same semester so math should be ok. One thing I did really poorly in was stoichemetry, so if somebody can explain the basic concepts to that to me in plain english it would help me a lot. If anybody has any resources, facts and tid bits, or advice about the test or the course I would appreciate that as well. Anything to help me pass . Thanks in advance.

 

[CycloWizard]

Stoichiometry is simply ensuring that the number of atoms of each element is conserved through a reaction: if you have 6*H2O on the left side, then you had better end up with 12 hydrogen atoms and 6 oxygen atoms on the right side. There are plenty of stupid tricks to find the number of each molecule which makes this happen, but this is the basis of it and it's rare that it's ever difficult to find them by inspection.

 

[Mothergoose729]

dp

 

[Mothergoose729]

Stoichiometry is simply ensuring that the number of atoms of each element is conserved through a reaction: if you have 6*H2O on the left side, then you had better end up with 12 hydrogen atoms and 6 oxygen atoms on the right side. There are plenty of stupid tricks to find the number of each molecule which makes this happen, but this is the basis of it and it's rare that it's ever difficult to find them by inspection.

Ah yes, I can feel it coming back to me. I remember proving that octane broke down into carbon dioxide and water. I think I started to fall off when grams and moles where thrown into the mix though. Something about converting messy units in my clean math made me uneasy.

 

[CycloWizard]

Ah yes, I can feel it coming back to me. I remember proving that octane broke down into carbon dioxide and water. I think I started to fall off when grams and moles where thrown into the mix though. Something about converting messy units in my clean math made me uneasy.

Just remember that units cancel just like numbers do and write it out, like

2 moles H2O*(2 moles H2/1 mole H20)*(2.0158 g/1 mole H2)=8.0632 g.

As long as you use a systematic approach, you will always arrive at the right answer. You might want to figure out what approach you want to use in case you have a time constraint on the exam.

I took an exemption exam for general chemistry my freshman year of college. I got just about everything right, with the notable exception of every calculation which used Avagadro's number. I hadn't taken chemistry in three years and couldn't remember it for the life of me, so I automatically missed all of those questions. Now I know it off the top of my head to a pretty high degree of accuracy...

 

[tcsenter]

I took an exemption exam for general chemistry my freshman year of college. I got just about everything right, with the notable exception of every calculation which used Avagadro's number.

I love guacamole!

 

[A_Dying_Wren]

Basically grams per mole = grams/moles and moles = grams/(grams/mol). Don't forget to get your molar ratios right. You will probably need to remember PV=nRT as well so refresh up on the gas constant. There's also fun stuff like calculating concentrations which shouldn't be too hard. ALWAYS write out the equation first if it isn't given to you. Make sure the oxidation numbers balance and whatnot.

Stoich really isn't hard but i dunno how much you americans do in school.

 

[tcsenter]

Stoich really isn't hard but i dunno how much you americans do in school.

As much as you want to take. Anything beyond an 8th grade introduction to chemistry (that is usually combined as part of a more general science class) is elective.

 

[Mr. Pedantic]

Probably the best advice about stoichiometry is that it's vitally important that you get the units right, and keep them in mind throughout the whole calculation. Otherwise you jot down all these random numbers from your calculation and at the end when you do the final thing you can't remember what any of them mean or where they came from.

 

[jimhsu]

Ah, stoichometry. Brings back memories of high school chemistry...

Upper div college chemistry is way different though.
Rule #1: No one outside an academic/professor setting ever truly understands thermodynamics, and everything you learned at high school about it is most likely wrong (i.e. entropy != disorder, unlike what they teach you at high school).
Rule #2: If that quantum mechanics problem seems too easy, most likely you're doing it wrong.

I could care less about thermodynamics, but in at least some endeavors it is important (i.e. preventing your 10 story tall industrial reactor from exploding). I couldn't care less about the academic exposition part of it though.