Chemistry
posted by Steven .
A 25.40 g sample of water containing tritium,(3/1)H , emits 1450 beta particles per second. Tritium is a weak beta emitter, with a halflife of 12.3 yr. What fraction of all the hydrogen in the water sample is tritium?

I would convert 12.3 years to seconds, then k = 0.693/t_{1/2}.
rate = 1450 dps = kN
Solve for N = number of atoms.
Then determine the fraction that is of 25.40 g H2O H atoms. Remember H2O has two H atoms.