, atomic mass = 239 u, decays by decay with a half-life of 24,100 y. We have to calculate the amount of helium in grams produced by an initially pure 12.0-g sample of after 20,000 y. We recall that particle is a helium nucleus and has an atomic mass of 4.00 u.

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, atomic mass = 239 u, decays by decay with a half-life of 24,100 y. The decay constant of will therefore be

The atomic mass of is 239 u. Therefore, the number of nuclides in a 12.0 g sample of pure will be

In 20,000 y the number of particles produced by radioactive decay of nuclides would be given by

As particle is a helium nucleus and has an atomic mass of 4.00 u the amount of helium produced by 12.0 g of in 20,000 y will therefore be