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Homework Help: Stoikiometry for gun powder

  1. May 16, 2005 #1
    I need some help on how to find out how many grams of each substance i need to make gun powder for my Chemisty class. I have the equation its 2KNO3+3C+S -->3CO2 + K2S+ N2 I know that there are 10 grams but i have to figure out how many grams are for each substance. I'd just like someone point me in the right direction. I'm not sure what to do first. I know i need to use stoikiometry to figure it out. but if someone could just give me a little advice on what to do first. i would appreciate it a lot. thank you. as far as i know i have balanced the equation correctly but if someone sees a mistake, let me know please. thanks again.

  2. jcsd
  3. May 17, 2005 #2


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    I do hope this is purely an academic exercise and you are not trying to actually make this stuff. A balanced equation tells you how many atoms of each element are involved in the reaction. If you have exactly the right ratios for the number of atoms, you will have a complete reaction. Since atoms of different elements have different weights, knowing the ratios of the numbers is not the same as knowing the ratios of the weights, but if you know the number ratios, and the weights of each atom, you can figure out the ratios of weights.

    For example, the reaction for water is

    2H2+O2 -->2H2O

    4 hydrogen atoms and 2 oxygen atoms combine to form two water molecules. In atomic mass units (amu), the weight of hydrogen is about 1 and the weight of oxygen is about 16. If A is some unspecified multiplying factor measured in grams per amu, you would need to have

    A*4*1amu + A*2*16amu = grams of water formed.

    You could sove this equation for A. Then you could find the weight of each element needed for the reaction.
  4. May 17, 2005 #3
    Oh geez.. you totally confused me with everything u said. and yes i am making gun powder @ school in my chemistry class- that is if i figure out the equation. the amu stuff..yeah idk what ur talking about. all i know if that we have to figure out the moles, the g/mol, and the mass. maybe u could put it in lamens terms so i can understand? thanks for the attempt though- sorry that im dumb lol.
  5. May 17, 2005 #4


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    In terms of moles and grams per mole then- The reaction for water is

    2H2+O2 -->2H2O

    2 hydrogen molecules and 1 oxygen molecule combine to form two water molecules. In terms of moles, 2 moles of hydrogen molecules and one mole of oxygen molecules combine to form two moles of water molecules. If A is some unspecified multiplying factor, you would need to have

    A*(2 moles hydrogen)*(2 grams/mole) + A*(1 mole oxygen)*(32 grams/mole) = A*(2 moles water)*(18 grams/mole) = # grams of water formed.

    If A = 1 you would get 4 grams hydrogen + 32 grams oxygen --> 36 grams of water (2 moles)

    If A = 1/2 you would get 18 grams of water (1 mole)

    If you wanted 10 grams of water, you would find the value of A that will give you 10 grams of water. That would be

    A = 10 grams/(4 grams + 32 grams) = 5/18

    So you would need (5/18)*2 moles of hydrogen or 5/9 moles and (5/18)*1 moles of oxygen or 5/18 moles and the result would be

    (5/18)*(2 moles hydrogen)*(2 grams/mole) + (5/18)*(1 mole oxygen)*(32 grams/mole) = 10 grams of water

    So now you know how many moles of hydrogen you need, and you can figure out how many grams that is because you know there are 2 grams per mole. You also know how many moles of oxygen you need and you can figure out haw many grams that is.

    Follow the same steps for your reaction, and you will arrive at the number of grams of each compound needed. In your problem you have to also take into consideration that the product of the reaction includes other compounds besides the one you are trying to produce, so not all of the ingredients are turned into desired products. You should be able to figure out how to extend the water example to the more general case.
    Last edited: May 17, 2005
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