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# Rate constant question watch

1. 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?
2. (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
3. (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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