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    Compound A undergoes a first order chemical reaction according to the equation
    ln([At]/[Ao]) = -kt
    After 145 s, 41.2% of A has been consumed.
    Determine the rate constant k.

    ln(41.2/100) = -k (145)

    -0.887 = -k (145)

    -0.887/145 = -k

    6.115x10^-3 = k

    Calculate the overall time needed for the 70% of A to be consumed.
    ln(70/100) = (-6.115x10^-3)t
    t = 58.33

    I thought i was doing the first part right but then the second part doesnt make sense. Can anyone help with this question please?
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    (Original post by elef95)
    Compound A undergoes a first order chemical reaction according to the equation
    ln([At]/[Ao]) = -kt
    After 145 s, 41.2% of A has been consumed.
    Determine the rate constant k.

    ln(41.2/100) = -k (145)

    -0.887 = -k (145)

    -0.887/145 = -k

    6.115x10^-3 = k

    Calculate the overall time needed for the 70% of A to be consumed.
    ln(70/100) = (-6.115x10^-3)t
    t = 58.33

    I thought i was doing the first part right but then the second part doesnt make sense. Can anyone help with this question please?
    The mistake you've made here is subtle but makes a big difference to the outcome. Note the A(t) is the amount of A that is remaining at time t, not the amount that was consumed. Hence if 41.2% was consumed after 145s then 58.8% is remaining.

    So your methodology is correct, you've simply used the wrong values for A(t).

    Hope this helps
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    (Original post by phyzard)
    The mistake you've made here is subtle but makes a big difference to the outcome. Note the A(t) is the amount of A that is remaining at time t, not the amount that was consumed. Hence if 41.2% was consumed after 145s then 58.8% is remaining.

    So your methodology is correct, you've simply used the wrong values for A(t).

    Hope this helps
    Thank you!
 
 
 
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