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Help on science please

I need help with these questions:
 
1. Use the reaction that occurs when magnesium burns in oxygen to show how a reaction might be included in more than one category of reaction.
 
2. Explain how energy is conserved in an endothermic reaction.
 
Also, can someone please explain how to calculate moles, how to calculate molar mass, and all the steps for different kinds of mole problems?
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1 Answer

 
1.  Well. the best way to look at your first question is to write out the chemical equation in either word form or chemical form so that you can look for hints for the types of reactions that are occurring.
 
magnesium + oxygen gas -->  magnesium oxide
 
Since this reaction is in the presence of oxygen and burns, we can jump to the conclusion that there is fire or heat produed.  Right?  Do you know what kind of reaction that is?
 
This reaction also shows that there are 2 reactants and 1 product ...you are making one thing out of two or more things.  Do you know what type of reaction that is?
 
2.  First, you need to think about what conservation is?  If energy is conserved, can it be created or destroyed?  Can it be changed into a different type of energy?    Next, you need to think about what an endothermic reaction is... What does endo mean?  What does therm mean?  Finally, you can put your answers together...if energy can not be lost, and energy is going into this reaction, where could the energy possibly be going?
 
3.  I can help you will calculating moles, molar mass, and kinds of mole problems....Is there a particular problem you would like help with as an example? 

Comments

I'd like help on these problems with moles:
1. What mass of KBr is contained in 2.50 moles of the compound?
2. How many moles of Na2CrO4 are contained in 74.3 grams of the compound?
 
Thank you for your help :)
 
You are very welcome.
Problem one:
 
1. The first step to any problem is to identify what you know and what you want to know.
for this problem...you know you have 2.5moles and you want the mass...
2. The next step is to look at your pathway....when looking at one type of compound (you are not moving between compounds, for example not going from flour to cookies (if you're cooking) or from KBr to CO2 in chem)...the pathways are always grams to moles to atoms (moles are always in the middle, think m for middle).
so you are going from moles ---to---> grams
3. Next you need conversion units.  For every arrow or step you are moving you need 1 conversion unit.  so for this problem you need to know how many grams of KBr are in 1 mole of KBr.... you ALWAYS go out of 1 mole as the periodic table is based on one mole!  Since, you can't find KBr on the table as its a compound, you need to add up the pieces of KBr to get the mass in grams in one mole....
So, looking at the table... you find K and then Br....K has a mass of 39.10 (and you one have one K in the chemical formula) and Br has 79.90 grams.  Notice we are using the atomic mass on the periodic table to equal the number of grams there is in one mole.... now add these together....you get 119.0g/1 mole.... this is your conversion unit.  (*if your formula had more than 1 of each, you would need to multiple, such as in your second problem...you would need to add 2 Na's, 1 Cr, and 4 O's to get your mass).
4.  now I have my students make a table that looks like  this    ________|__________
                                                                                                          |                   
 
This is the FIRST time you ever look at the numbers given in the original problem and you fill in the boxes....
the top left hand box is where you put the given number with the units, see below.  the bottom left hand box is ONLY used if you are doing compound units like meters/seconds, so I have my students put a big X in it if its not being used, see below.  Then you figure out how to put in your conversion unit.  you want to cancel out the units you DON't want, so in this case the moles.  Make sure you put the units in so you can cancel.  Picture this as a math problem were the horizontal line is a division bar where you can cancel things out.  See below. Then multiply all the way across the top and divide by the whole bottom! And you are done!                         
 __2.5 mol__|   119g   = (2.5)(119)G    =  297.5 g KBr
    X             |1 mol               1
 
 
Do you think you can try umber 2 yourself?  You are just working the other way.  You should get around 0.4mol depending on how you round.
Okay my table looks off... let me just post a better first picture for you, 
 
 
 
                |
_________|___________
                |
                |
 
Hope that looks a bit better- not sure how to do it better without drawing it.
And I have another question:
Is the atomic mass of an element equal to its molar mass?
atomic mass is equal to the molar mass... you use Avogadro's number if you are want to know how many atoms there are or they give you how many atoms there are.  and Avogadro's number is ALWAYs over 1 mole for your unit conversion.  for the second problem you are only going from moles to grams, no need to go to atoms...remember moles is the middle man.