AP Chemistry 3.5 Chemical Reaction Rates
AP Chemistry 3.5 Chemical Reaction Rates. How much 85Kr will we need to collect?
|AP Chemistry||Chemical Elements Are Building Blocks|
Chemical Reaction Rates
|Test Prep||AP Chemistry|
can render Superman powerless. [Krptonite and Superman appears]
We learn that we need 18 g of a radioactive isotope of krypton, krypton 85, to complete
our recipe for world domination.
Krypton 85 has a half-life, t one half, of about 10 years, and it only exists on the
It will take us 20 years to return from Neptune back to Earth.
How much krypton 85 will we need to collect from Neptune to have enough left to take over the world?
And here are your potential answers:
Superman is strong, fast, and pretty darn handsome, but even he can be defeated by a [Superman makes robbers disappear]
chemical compound. [Masked man shows Superman kryptonite]
So you can feel better about AP Chem being your one weakness.
You’re in good company.
To solve today’s problem, we need to remember that the half-life is the amount of time it
takes for half of the radioactive compound krypton 85 to decay.
After each half-life passes, the amount of krypton 85 left has decreased by a factor
Not to be mixed up with the time period “half of your life,” which is how long we were [Man sat waiting at a table]
waiting for Adele to release her new album. [Adele singing]
No hard feelings, Adele.
It was worth the wait.
Anyway, the half-life of krypton 85 is 10 years, but the journey back from the planet
Neptune will take 20 years.
It will also take us 20 years to get to Neptune…
Forty years is a perfectly reasonable amount of time to take to surprise somebody, right? [Superman grows a beard]
Nobody can catch on when you’re moving that quickly!
So the 20 year journey home is equal to two half-lives of Krypton 85.
During this time, the amount of krypton 85 will be cut in half twice, so the amount remaining [Masked man holding krypton]
when we get back to Earth will be only one quarter of the amount we had when we left
We’ll have to gather exactly four times as much as we need, but absolutely no more. [A small and large piece of krypton]
We might be evil, but we don’t have to be wasteful. [Masked man drops soda can in the trash]
Since we’ll need 18 grams of krypton 85 on earth, and we have to gather four times
as much on Neptune, that means we have to collect 4 x 18 = 72 grams.
That means D is the correct answer.
Now all that’s left is to execute our brilliant secret plan.
Good thing Superman doesn’t watch Shmoop. [Superman watching Shmoop on TV]
Or does he…? Dang it.