halflife
posted by Issac on .
1. The halflife of a firstorder reaction is 9.89 minutes. How many minutes will have elapsed after 27 halflives?
My answer is 267.03.
2. The halflife of a secondorder reaction is 2.508 days. How many days will have elapsed after 8 halflives?
My answer is 5.330.
Is this correct or did I do it wrong?
PLEASE HELP ME!!!!
The first one is ok except you have only three signifant figures so you should report to 267.
I don't get your answer for #2. For a second order reaction, The first half life occurs at 2.508 days. The second half life is 2x that or 5.016 days, the third half life is 4x that or 10.032 days etc.

Rn222 has a halflife of 3.82 days. How long before only 1/16 of the original sample remains?
Solution:
recognize 1/16 as a fraction associated with 4 halflives (from (1/2)4 = 1/16)
3.82 days x 4 = 15.3 days 
U238 has a halflife of 4.46 x 109 years. Estimates of the age of the universe range from 9 x 109 years to 23 x 109 years (Cauldrons in the Cosmos: Nuclear Astrophysics, C.E. Rolfs and W.S. Rodney, Univ. of Chicago, 1988, p. 477). What fraction of this isotope present at the start of the universe remains today? Calulate for both minimum and maximum values, as well as a median value of 16 x 109 years.
Solution:
1) Calculation for the median value:
(16 x 109) / (4.46 x 109) = 3.587 halflives
2) What fraction remains?
(1/2)3.587 = 0.0832
8.32% remains 
The activity remaining in a 3mCi source of Cs137 after 3halflives (30 years) have elapsed; Cs137 halflife is 30.17 years.