Iodine-131 decays with a half-life of 8.02
d. in a sample initially containing 5.00 mg of 131i, what mass remains after 6.01 d

Radioactive material undergoes 1st order decay kinetics.

For 1st order decay, half life = 0.693/k

where k = rate constant

k = 0.693/half life = 0.693/8.02 = 0.0864 day-1

Now, for 1st order reaction,
k = [tex] \frac{2.303}{t} X log \frac{initial.conc}{final.conc} [/tex]

Given: t = 6.01d, initial conc. = 5mg

∴0.0864 = [tex] \frac{2.303}{6.01} X log \frac{5}{final.conc} [/tex]
∴ final conc. = 2.975 mg