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Homework Help: Limiting Reagents With 3 Reactants

  1. Jul 15, 2012 #1
    1. The problem statement, all variables and given/known data

    So my test is tomorrrow, and I have troubles with limiting reagents, I can get them right sometimes with the 2 reactant questions, but not always with 3.... so the question is

    Q: What mass of phosphorus (P4) is produced when 41.5g of calcium phosphate, 26.5g of silicon diioxide, and 7.80g of carbon are reacted according to the eqation:

    2Ca3(PO4)2 + 6SiO2 + 10C -> P4 + 6CaSiO3 + 10CO

    2. Relevant equations


    3. The attempt at a solution

    I've checked that the equation is balanced
    and I calculated the moles of 2Ca3(PO4)2 which is 0.1337 mols, 0.441 mols for 6SiO2 and 0.0649 mols for 10C.

    after this idk what ratios to use to calculate what....
  2. jcsd
  3. Jul 17, 2012 #2


    User Avatar

    Staff: Mentor

    No matter how many reactants, the correct answer is always the one giving the lowest amount of products.
  4. Jul 18, 2012 #3
    Ok. First, convert all the grams given into moles.

    Then, write the ratio of theoretical moles to actual moles for each compound. Finally, for each ratio, divide the whole ratio by its denominator so all the ratios have a denominator of 1. Then see which is the lowest.

    Assuming you have all the grams to moles converted properly, you get this:

    0.06685/1 Ca3(PO4)2

    0.0735/1 SiO2

    0.00649/1 C

    Since 0.00649 is the least for 1 mol, then Carbon is the limiting reagent.
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