So 6.02*10^22 is avogadro constant, which is the amount of atoms in one mole. If you look Xenon up in the periodic table you will find it's mass given <span>131,293, which is grams per 1 mole.
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Radioactive material obeys 1st order decay kinetics,
For 1st order reaction, we have
k =

where, k = rate constant of reaction
Given: Initial conc. 100, Final conc. = 6.25, t = 18.9 hours
∴ k =

= 0.1467 hours^(-1)
Now, for 1st order reactions: half life =

= 4.723 hours.