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    If, during a first order reaction, the concentration fell to 12.5% of its original value in 24 minutes, what is the half life of the reaction?
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    It's not really that difficult.
    Think about it - there is a 12.5% decrease in 24 minutes so how long will it take for it to have a 50% decrease.

    12.5% = 24
    50% = ?

    You need to do 50/12.5 and with that answer multiply it to 24
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    (Original post by Namita Gurung)
    It's not really that difficult.
    Think about it - there is a 12.5% decrease in 24 minutes so how long will it take for it to have a 50% decrease.

    12.5% = 24
    50% = ?

    You need to do 50/12.5 and with that answer multiply it to 24
    I got 96 min but the answer in my book is 8 min?!
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    You first have to find out how many half lifes there is overall. So you can keep halving from 100 until you get to 12.5. 100 - 50 - 25 - 12.5 which equates to 3 half lifes because it has been halved 3 times. So now you know there are 3 half lifes in 24 minutes so to find the length of one you do 24/3 which = 8 minutes and therefore one half life is 8 minutes. Hope that helped!
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    (Original post by Samii123)
    You first have to find out how many half lifes there is overall. So you can keep halving from 100 until you get to 4. 100 - 50 - 25 - 12.5 which equates to 3 half lifes because it has been halved 3 times. So now you know there are 3 half lifes in 24 minutes so to find the length of one you do 24/3 which = 8 minutes and therefore one half life is 8 minutes. Hope that helped!
    Oh i see, thank you!
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    (Original post by Adorable98)
    Oh i see, thank you!
    You're welcome
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    (Original post by Samii123)
    You're welcome
    I've got one more question please.

    If the initial concentration in a first order reaction [would it be any different if it was a zero or a second order reaction??] with a half life of 12 minutes was 0.400 moldm-3, what would the concentration be after 48 minutes?
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    (Original post by Adorable98)
    I've got one more question please.

    If the initial concentration in a first order reaction [would it be any different if it was a zero or a second order reaction??] with a half life of 12 minutes was 0.400 moldm-3, what would the concentration be after 48 minutes?
    With a half life of 12 minutes, in 48 minutes there would be 4 half lifes that have occurred as 48/12 = 4. So then you must half the concentration a total of 4 times. 0.400 - 0.200 - 0.100 - 0.05 - 0.025. So your final answer is 0.025moldm^-3.
    Also first order reactions are when the rate of reaction is directly proportional to the change in one reactant and therefore half life can be calculated accurately. In a second order reaction the rate depends on two substances and therefore half life cannot be calculated as accurately and it becomes a more complex calculation.
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    (Original post by Samii123)
    With a half life of 12 minutes, in 48 minutes there would be 4 half lifes that have occurred as 48/12 = 4. So then you must half the concentration a total of 4 times. 0.400 - 0.200 - 0.100 - 0.05 - 0.025. So your final answer is 0.025moldm^-3.
    Also first order reactions are when the rate of reaction is directly proportional to the change in one reactant and therefore half life can be calculated accurately. In a second order reaction the rate depends on two substances and therefore half life cannot be calculated as accurately and it becomes a more complex calculation.
    I see thank you so much!
    So do you think we'll only be asked half-life questions for 'a first order' at A2?
    I'm doing Edexcel.
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    (Original post by Adorable98)
    I see thank you so much!
    So do you think we'll only be asked half-life questions for 'a first order' at A2?
    I'm doing Edexcel.
    I'm not sure, you could have a look at the specification however.
 
 
 
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