The radionuclide has a half-life of 12.7 h. We have to find how much of an initially pure 5.50-g sample of will decay during the 2-h period beginning 14.0 h later.

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The radioactive decay law is

,

where is the number of radioactive nuclides present at . It is given that the half-life of is 12.7 h. Its decay constant will be

As the number of nuclides in a pure sample is proportional to the mass of the sample, the amount of radionuclide from a sample of 5.50 g that would remain after 14.0 h will be

Therefore, of the 2.56 g of radionuclide that will remain after 2 h of radioactive decay will be

Therefore, of an initially pure 5.50-g sample of that will decay during the 2-h period beginning 14.0 h later will be