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RavenGod
April 30th, 2016, 08:43 PM
I have a chemistry final coming up and I was sick for almost all of the subject earlier this year and I was hoping someone could direct me to a site that starts from the basics to Stoichiometry and also how to balance equations. Trust me, I do NOT want to have to repeat my sophomore year. Once is enough for me lol. Thanks in advance by the way :)

Microcosm
May 1st, 2016, 12:59 PM
RavenGod,

I am a sophomore in high school taking Chemistry as well. We have already done stoichiometry--we finished it a couple of weeks ago. I got a 92 on the Stoich test so I think I can help. In this post, I will review everything I was taught about stoichiometry and provide some resources for you throughout my explanation. It's going to be a lot to take in, so please study the post carefully, bit by bit. I'll be using my actual chem notes as a guide such that I make minimal mistakes.

There are two kinds of stoichiometry: composition stoichiometry and reaction stoichiometry.

I'll start with how to do composition stoich:

Composition stoichiometry is defined as follows: a type of stoichiometry which deals with the mass relationships of elements in compounds.

For starters, you'll need to know Avogadro's number, which is 6.022 x 10^23. This number represents the number of atoms in a mole and is "the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state" (Source (http://www.americanscientist.org/issues/pub/an-exact-value-for-avogadros-number)).

Next, you'll need to know about molar mass, which is the amount of grams present in one mole of any substance. As such, it is measured in terms of grams per mole, written as g/mol. For any given element, this number can be obtained by looking at the periodic table. For instance, oxygen's molar mass is 16 grams per mole.

The molar mass of a compound can be found by adding up the molar masses of its component elements. Here's an example of that:

Find the molar mass of NaOH (Sodium hydroxide).

NaOH
Na = 22.99g/mol
O = 16g/mol
H = 1.008g/mol
Molar mass of compound = 39.998g/mol



There is another important component of composition stoichiometry which you ought to know as well: percent composition. Percent composition is the percent mass of one element in a compound. It can be found using this formula:


( Mass of one element / total mass of compound ) x 100 = Percent composition of element.

Mass of one element
_____________________ x 100

total mass of compound


To make this concept clearer, here is an example of finding the percent composition of an element in sodium hydroxide:


Find the percent composition of sodium in one molecule of NaOH.

Molar mass(as calculated above): 39.998g/mol
Molar mass of only sodium(Na): 22.99g/mol

Now, we use our formula:

22.99g/mol
____________ x 100 ≈ 57% <- This is rounded to the nearest whole number.

39.998g/mol


For the sake of not making this post too long, I'll omit the explanation of empirical and molecular formulas as well as hydrate composition. However, I will direct you to some pages that explain the concepts. A quick google search should answer most questions you have on them.


Empirical and molecular formulas:
http://www.kentchemistry.com/links/bonding/empirical.htm
https://www.youtube.com/watch?v=wnRaBWvhYKY
Note: I strongly urge you to watch more of Tyler Dewitt's videos. They will help you tremendously in chem.

Hydrates:
https://www.youtube.com/watch?v=cOFDkeV2yO4
https://www.youtube.com/watch?v=NFCXn466Z7g
Note: A hydrous compound is one which contains the water and an anhydrous compound is one from which water has been removed.

And that's about it for composition stoich for general chemistry.


And now, moving on to our second form of stoichiometry: Reaction stoichiometry.


Reaction stoichiometry can be defined as a type of stoichiometry which involves mass relationships between reactants and products in a reaction.

Note that a reaction is written like so:

Reactant + Reactant => Product + Product

Reaction stoichiometry depends on the Law of conservation of mass, which states that, in any chemical reaction, mass is not created nor destroyed, but may change state.

To solve a stoich problem:

1.) You must first have a balanced chemical equation.
2.) Find the mol ratios which will act as your conversion factors.
3.) Do mass to mole conversions.

Ex.:

Balanced chemical equation: 2Al + 6HCl => 3H2 + 2KOH

Next, you'd find your mole ratios depending on what the question is asking you. For instance, if the question gives you some mass of Aluminum which reacts and asks you how much HCl must react with it, you'd do as follows:

The mole ratio from Aluminum to Hydrochloric acid(HCl) is 2:6. This is obtained from the coefficients of your balanced chemical equation.

Now you'd do mole conversions. If you don't know how to do mole conversions, here (https://www.youtube.com/watch?v=CMnkSb2YsXI) is Tyler Dewitt explaining it.

To convert between reactants and products produced, you'll have to understand the limiting reagent concept and the excess reagent concept.


The limiting reagent is the reactant which determines the amount of product formed because, once all of its provided mass has reacted, the reaction can no longer continue.

The excess reagent is the substance that is not used up completely in the reaction, hence its name: excess. There is an excess amount left over after the reaction has occurred.

To find the limiting reagent, the problem must provide you with the masses of both reactants.

To find the limiting reagent in a reaction:
1.) Convert the masses of the reactants to moles using their molar masses.
2.) Convert moles of each reactant to moles of one of the products(be consistent when choosing a product to compare with both reactants).
3.) The reactant which yields the lowest moles of product is the limiting reagent.

To best visualize this, an example is necessary. However, it's difficult to provide an example here, so I will direct you to a YouTube video which finds the limiting reagent: https://www.youtube.com/watch?v=nZOVR8EMwRU

At about ten minutes in, Dewitt works an example problem. In the first ten minutes of the video, he explains the concepts in more depth than I have done here.




Good luck on your exams! Remember to watch Dewitt's videos if you need more help. They are very good.

TheFutureDoctor
May 7th, 2016, 09:16 AM
I have a chemistry final coming up and I was sick for almost all of the subject earlier this year and I was hoping someone could direct me to a site that starts from the basics to Stoichiometry and also how to balance equations. Trust me, I do NOT want to have to repeat my sophomore year. Once is enough for me lol. Thanks in advance by the way :)

Hello I'd suggest Khan Academy they are nonprofit and have a huge range. All the best!

ValentinClarke
May 7th, 2016, 09:18 AM
RavenGod,

I am a sophomore in high school taking Chemistry as well. We have already done stoichiometry--we finished it a couple of weeks ago. I got a 92 on the Stoich test so I think I can help. In this post, I will review everything I was taught about stoichiometry and provide some resources for you throughout my explanation. It's going to be a lot to take in, so please study the post carefully, bit by bit. I'll be using my actual chem notes as a guide such that I make minimal mistakes.

There are two kinds of stoichiometry: composition stoichiometry and reaction stoichiometry.

I'll start with how to do composition stoich:

Composition stoichiometry is defined as follows: a type of stoichiometry which deals with the mass relationships of elements in compounds.

For starters, you'll need to know Avogadro's number, which is 6.022 x 10^23. This number represents the number of atoms in a mole and is "the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state" (Source (http://www.americanscientist.org/issues/pub/an-exact-value-for-avogadros-number)).

Next, you'll need to know about molar mass, which is the amount of grams present in one mole of any substance. As such, it is measured in terms of grams per mole, written as g/mol. For any given element, this number can be obtained by looking at the periodic table. For instance, oxygen's molar mass is 16 grams per mole.

The molar mass of a compound can be found by adding up the molar masses of its component elements. Here's an example of that:

Find the molar mass of NaOH (Sodium hydroxide).

NaOH
Na = 22.99g/mol
O = 16g/mol
H = 1.008g/mol
Molar mass of compound = 39.998g/mol



There is another important component of composition stoichiometry which you ought to know as well: percent composition. Percent composition is the percent mass of one element in a compound. It can be found using this formula:


( Mass of one element / total mass of compound ) x 100 = Percent composition of element.

Mass of one element
_____________________ x 100

total mass of compound


To make this concept clearer, here is an example of finding the percent composition of an element in sodium hydroxide:


Find the percent composition of sodium in one molecule of NaOH.

Molar mass(as calculated above): 39.998g/mol
Molar mass of only sodium(Na): 22.99g/mol

Now, we use our formula:

22.99g/mol
____________ x 100 ≈ 57% <- This is rounded to the nearest whole number.

39.998g/mol


For the sake of not making this post too long, I'll omit the explanation of empirical and molecular formulas as well as hydrate composition. However, I will direct you to some pages that explain the concepts. A quick google search should answer most questions you have on them.


Empirical and molecular formulas:
http://www.kentchemistry.com/links/bonding/empirical.htm
https://www.youtube.com/watch?v=wnRaBWvhYKY
Note: I strongly urge you to watch more of Tyler Dewitt's videos. They will help you tremendously in chem.

Hydrates:
https://www.youtube.com/watch?v=cOFDkeV2yO4
https://www.youtube.com/watch?v=NFCXn466Z7g
Note: A hydrous compound is one which contains the water and an anhydrous compound is one from which water has been removed.

And that's about it for composition stoich for general chemistry.


And now, moving on to our second form of stoichiometry: Reaction stoichiometry.


Reaction stoichiometry can be defined as a type of stoichiometry which involves mass relationships between reactants and products in a reaction.

Note that a reaction is written like so:

Reactant + Reactant => Product + Product

Reaction stoichiometry depends on the Law of conservation of mass, which states that, in any chemical reaction, mass is not created nor destroyed, but may change state.

To solve a stoich problem:

1.) You must first have a balanced chemical equation.
2.) Find the mol ratios which will act as your conversion factors.
3.) Do mass to mole conversions.

Ex.:

Balanced chemical equation: 2Al + 6HCl => 3H2 + 2KOH

Next, you'd find your mole ratios depending on what the question is asking you. For instance, if the question gives you some mass of Aluminum which reacts and asks you how much HCl must react with it, you'd do as follows:

The mole ratio from Aluminum to Hydrochloric acid(HCl) is 2:6. This is obtained from the coefficients of your balanced chemical equation.

Now you'd do mole conversions. If you don't know how to do mole conversions, here (https://www.youtube.com/watch?v=CMnkSb2YsXI) is Tyler Dewitt explaining it.

To convert between reactants and products produced, you'll have to understand the limiting reagent concept and the excess reagent concept.


The limiting reagent is the reactant which determines the amount of product formed because, once all of its provided mass has reacted, the reaction can no longer continue.

The excess reagent is the substance that is not used up completely in the reaction, hence its name: excess. There is an excess amount left over after the reaction has occurred.

To find the limiting reagent, the problem must provide you with the masses of both reactants.

To find the limiting reagent in a reaction:
1.) Convert the masses of the reactants to moles using their molar masses.
2.) Convert moles of each reactant to moles of one of the products(be consistent when choosing a product to compare with both reactants).
3.) The reactant which yields the lowest moles of product is the limiting reagent.

To best visualize this, an example is necessary. However, it's difficult to provide an example here, so I will direct you to a YouTube video which finds the limiting reagent: https://www.youtube.com/watch?v=nZOVR8EMwRU

At about ten minutes in, Dewitt works an example problem. In the first ten minutes of the video, he explains the concepts in more depth than I have done here.




Good luck on your exams! Remember to watch Dewitt's videos if you need more help. They are very good.

How old are you, that you have to do this in high school?

Microcosm
May 7th, 2016, 10:43 AM
How old are you, that you have to do this in high school?

15, almost 16. It's a general chem class. At my school, there is just a general level(which is pretty much like honors level chem, it counts as an honors course in GPA), a Pre-AP course, and AP Chem. I'm taking AP next year.

ValentinClarke
May 8th, 2016, 04:22 AM
15, almost 16. It's a general chem class. At my school, there is just a general level(which is pretty much like honors level chem, it counts as an honors course in GPA), a Pre-AP course, and AP Chem. I'm taking AP next year.

Whoa... I'm 15 turning 16 in 5 months, and we are not doing this stuff until I am 17, I think , and in College, or maybe even in university...
I wish we could do AP Courses here in England...
Valentin

TheFutureDoctor
May 8th, 2016, 08:04 AM
Well Microcosm in my country I think this is done in XIth or XIIth (16 or 17). Woah sounds pretty complicated esp. as I am going into XI this year.