Please explain..chem ?

[tissy]

If the half-life of an isotopic nuclide is 24 hours, how long will it take for 75% of the sample to decay?



A. 12 hours
B. 24 hours
C. 36 hours
D. 48 hours
E. 60 hours

Correct Answer: D

Dont understand..I can do the simpler ones with no problem but for some reason I can do this one

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[thefifthbeatle]

In 24 hours, half is gone.....in another 24 hours (48 total) half of the HALF is gone

or

1 half life -50% decayed
2 half life -50% of that ^ 50% is decayed...which is 25% of the original

so 75%

or

1st half life: (1/2) * 2nd half life: (1/2) = 1/4 remaining, 3/4 gone

 

[DMD to Be]

I just made up a number of grams. Say 100 grams. 75% of this is 25. So we need to work on it until we get to 25.

100 -------- 50 (This is one half life.)



50 --------- 25 (This is two half lives.)


And we have 25 grams left. This means 75 grams (or 75%) of the starting material is gone.

Since it takes 24 hours for each half life, 24+24 = 48.

Hope this helps.

 

[tissy]

Thanks everyone...I see it clearly..i'm such a dork...you guys are the best in the west..wait east..who cares...you all are the best