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Quick question about limiting reagent

Discussion in 'MCAT Discussions' started by christian15213, Mar 5, 2007.

  1. christian15213

    christian15213 Banned Banned

    Jul 28, 2006
    When figuring a balanced equation with different ratio:s... <<< lol,

    Are the ratio's always the indicator of the limiting reagent or is the actual grams per reactants to grams per products what I should be focusing on...

    LOL, chem has been a while and everything in the beggining of the EK book has been convienient to think ratio's are the answer but I don't think that is the total trick of the game... I think mols and grams has something to do with it... could anyone elaborate?
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  3. mc4435

    mc4435 7+ Year Member

    Oct 7, 2006
    You need to compare moles. So if you're given grams, convert to moles to compare.
  4. dr.kicia

    dr.kicia Senior Member 7+ Year Member

    Dec 11, 2003
    where the heart is***
    there is a little trick which i have no clue why is not used in US (i leaner it in hs in Poland)
    You do not need to convert gr to moles and it is acctually a very fast way to calculate what you need. You just use a proportion.
    Let's just see it on an example ( i am making up the numbers LOL)
    a simple reaction
    H2 + O2--> H2O
    and let's say they give you 5 grams of H2 and 10 grams of O2
    a)and they ask you to find out which one is limmiting reagent?
    b)how many moles of H2O will be formed?
    c) how manyy grams of O2 or H2 will be left after reaction is complete (of course assuming that the gasses cannot escape into the air from our system)?
    first thing you need to do is balance the equasion
    so you get

    2H2 + O2---->2H2O and write what you have given below the reactants
    5g 10g

    using proportion and few calculations:

    2 moles of H2= 2x2x1= 4 g
    and 1 mole of O2= 16X2=32 g

    now proportion using the reaction equasion:

    if 4g of H2----reacts with---> 32 g of O2
    the how many (xg)-----will react with--->10g
    now cross multiply x=(10x4)/32=1.25g
    this means that 1.25 g of H2 will react FULLY with 10 g of O2 which means that there will be H2 left but no more O2 for it to react with so that means that O2 is a limmiting reactant.
    It doesn't matter which one you choose to start the problem with either way you get the write answer.

    for bart b)
    u use the limmining reactant info and again from proportion
    you have 10 g to give you -----how many (x) grams of water
    if 32g------gives you-------------(2x18)=36
    cross multiply
    x=(10x36)/32=11.25 g of H2O will be produced.
    Now notice that using this method you can limit conversions
    Ltes say that they give you the reactants in moles lets say O is still our limmiting and we have 7 moles of it.
    now using proportion and stoichiometry of reaction
    if 1 mole of O2------>2 moles of H2O
    7 moles of O2------>x moles
    When using this method always remember about kipping things together ie. O2 under each other and relationships the same across the proportion so if you start with if g---->moles
    then you need to follow in the same manner which is g----> moles
    i think this is by far the easiest way to calculate things in chemistry. Sometimes using just two proportions you can calculate the whole problem that normally takes a page or so of math where you can make mistakes in every line (and trust me on it i mastered this technique to the point i can do stuff in my head). You can solve any stochiometry related problems using this thing. I mean ANY...and i am not kidding. So this is deffinetly worth knowing.
    Hope this helps.
    It would be easier to explain this in person since it is a very visual approach but i hope this will be enough. Any questions just ask
  5. booji

    booji Member 7+ Year Member

    Jun 23, 2005
    That method is interesting, but it is essentially the same as converting to moles, except it is done in a more indirect sense. Thanks for the input.
  6. wisguy

    wisguy 2+ Year Member

    Sep 26, 2006
    that seems more complicated than converting to moles.
  7. dr.kicia

    dr.kicia Senior Member 7+ Year Member

    Dec 11, 2003
    where the heart is***
    it does because i had to write out the explanation how to do it but all you really need to solve that problem is 2 proportions.
    It's fast and logical and you will never make any conversion error with this method.
  8. wisguy

    wisguy 2+ Year Member

    Sep 26, 2006
    yeah...most math is harder to write about than explain, so if it works for you go with it!
  9. dr.kicia

    dr.kicia Senior Member 7+ Year Member

    Dec 11, 2003
    where the heart is***
    yeah i love this method because it is almost imposible to make a conversion or math mistake. Unless you can't multiply two numbers and then devide the answear by another number LOL.
    It also limits the math since you do not need to convert everything. Let's say you had 5 g of reactant reacting to produce 0.003 moles of product and reaction is 1:1
    now you always know molar mass of reactant from PT and let's say it's 10
    all you need to do now is
    5g------0.003 moles
    10g------x moles
    x=(10x0.003)/5= 0.006 moles
    and you have your answear right away without even having to convert grams to moles first.
    For the MCAT this is worth knowing because as you may know converting all that stuff in your head (since we can't use calculators) would be kinda hard and time consuming. And we all know that during MCAT time is everything.
    I hope that this can help someone couse it sure does help me be more time efficient and prevents converting errors.
    You can also use this method for converting but you don't even have to.

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