Question

Radioactive materials are often used in biological studies. A radiation biologist studies the rate of decomposition of a certain substance and obtains the following data:

Time (days)	0	4.0	8.0	12.0	16.0
Mass (μg)	17.10	11.79	8.13	5.60	3.86

Determine the rate constant in units of days^-1.

How many micrograms will remain after 28 days?

Answer 1

K   = 2.303/t1-t0 log[A0]/[A]

      = 2.303/4-0 log17.1/11.79

     = 2.303*0.161/4    = 0.0927day^-1

K = 2.303/t log[A0]/[A]

0.0927 = 2.303/28 log17.1/[A]

log17.1/[A]   = 0.0927*28/2.303

log17.1/[A]   = 1.127

17.1/[A]        = 10^1.127

17.1/[A]       = 13.4

[A]               = 17.1/13.4 = 1.276microgram