What percentage of strontium-90 remains in a sample after three half-lives have passed? 12.5% 15% 25% 60%?

12.5%

Explanation:

The half life of any reaction is the amount of time it takes to consume half of the starting material.

In this case, the original or starting percentage of material is 100%

100% - 50% : First Half Life

50% - 25%   : Second Half life

25% - 12.5% : Third Half life.

After three half lives have passes, the percentage of strontium-90 that remains in a sample is 12.5%

the grams   of   barium that   are in   2.3 x 10^23   atoms of barium   is   52.4 g (answer c)

    calculation

mass = moles   of ba   x molar   mass of ba

  molar mass of ba from periodic table=   137.3 g/mol

  use the   avogadro's law   constant   to   calculate the moles of barium

  that   is     1 mole =   6.02   x 10^23 atoms

                    ? moles = 2.3 x 10^23   atoms

by cross   multiplication

=(1 mole   x 2.3x 10^23 atoms)   / 6.02   x10^23 atoms)=   0.382   moles

mass   is therefore = 137.3   g/mol x 0.382 mol = 52.4 ( answer c)

explanation: