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    Basically, when you are working out Kc concentration for products, you do initial moles + moles reacted when given an equilibrium concentration for a reactant, for example.

    So if a reactant was 1 mole intially, but 0.3 at equilbrium and you were finding out the equilibrium moles for product, would it be (0+0.3) or (0+0.7) ?
    Im getting confused.

    Thanks
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    LPresuming that you are saying the reactant started at 1 mol and was found to have 0.3 mols of it left at equilibrium:

    We can calculate a number of moles used up, x:

    1 - 0.3 = 0.7

    To calculate the number of moles of product at equilibrium we do:

    number of moles of product x mol of reactant used up

    so:

    NaCl ==> Na + 1/2Cl

    NaCl:
    Initial: 1.0
    Equilibrium moles: 0.4

    Moles reacted = 0.6

    so at equilibrium

    Cl: 0.3 moles

    Does that make sense?
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    (Original post by S.H.Rahman)
    LPresuming that you are saying the reactant started at 1 mol and was found to have 0.3 mols of it left at equilibrium:

    We can calculate a number of moles used up, x:

    1 - 0.3 = 0.7

    To calculate the number of moles of product at equilibrium we do:

    number of moles of product x mol of reactant used up

    so:

    NaCl ==> Na + 1/2Cl

    NaCl:
    Initial: 1.0
    Equilibrium moles: 0.4

    Moles reacted = 0.6

    so at equilibrium

    Cl: 0.3 moles

    Does that make sense?
    So should I always find out moles reacted? or only equilibrium moles to then find out conc to put into Kc.

    I thought Cl would be (0+0.4) then divide by two which is 0.2. So I should always use moles reacted value.
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    (Original post by PleaseHelppppp)
    So should I always find out moles reacted? or only equilibrium moles to then find out conc to put into Kc.

    I thought Cl would be (0+0.4) then divide by two which is 0.2. So I should always use moles reacted value.
    Yes, you should use your 'moles reacted x moles of product' to find your product moles at equilibrium. I would suggest using the ICEC method, which is what I, and everyone else in my school, use to do Kc questions. It really helps!

    Where:
    x - change in moles (moles reacted)

    I - Initial Moles
    C - Change
    E - Equilibrium moles
    C - Conc (divide by volume)

    e.g.

    NaCl ==> Na + 1/2Cl

    Q: 1 mole of NaCl decomposes in a container of volume 200cm^3. It is found that 0.4 moles of NaCl remain at equilibrium. Calculate Kc for this reaction.

    Write down what you know:
    NaCl | Na | Cl |
    I: 1 | 0 | 0 |
    C: 1-x | 0+x | 0 + 1/2x | <=== we only add half the change for Cl (1/2Cl)
    E: 0.4| | |


    NaCl | Na | Cl |
    I: 1 | 0 | 0 |
    C: 1-x | 0+x | 0 + 1/2x | <=== we only add half the change for Cl (1/2Cl)
    E: 0.4| 0.6 | 0.3 | <=== x must be 0.6 (1 - x = 0.4)
    C: [divide all above by 0.2]

    Kc = ([Na][Cl]^1/2) / ([NaCl])
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    (Original post by S.H.Rahman)
    Yes, you should use your 'moles reacted x moles of product' to find your product moles at equilibrium. I would suggest using the ICEC method, which is what I, and everyone else in my school, use to do Kc questions. It really helps!

    Where:
    x - change in moles (moles reacted)

    I - Initial Moles
    C - Change
    E - Equilibrium moles
    C - Conc (divide by volume)

    e.g.

    NaCl ==> Na + 1/2Cl

    Q: 1 mole of NaCl decomposes in a container of volume 200cm^3. It is found that 0.4 moles of NaCl remain at equilibrium. Calculate Kc for this reaction.

    Write down what you know:
    NaCl | Na | Cl |
    I: 1 | 0 | 0 |
    C: 1-x | 0+x | 0 + 1/2x | <=== we only add half the change for Cl (1/2Cl)
    E: 0.4| | |


    NaCl | Na | Cl |
    I: 1 | 0 | 0 |
    C: 1-x | 0+x | 0 + 1/2x | <=== we only add half the change for Cl (1/2Cl)
    E: 0.4| 0.6 | 0.3 | <=== x must be 0.6 (1 - x = 0.4)
    C: [divide all above by 0.2]

    Kc = ([Na][Cl]^1/2) / ([NaCl])
    Ok, i understand. Thanks
 
 
 
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