An unknown radioactive substance has a half-life of 3.20 hours . if 22.8 g of the substance is currently present, what mass a0 was present 8.00 hours ago? express your answer with the appropriate units.

129 g

Explanation:

The radioactive decay of a substance can be represented through the following expression.

where,

A is the amount of A at certain time t

A₀ is the initial amount of A

k is the rate constant

We can find k from the half-life (t1/2) using the following expression.

If 22.8 g of the substance is currently present, what mass A₀ was present 8.00 hours ago?

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:

atomic number of carbon is 6

electronic configuration of carbon is 1s²2s²2p²

orbital diagram of c is as shown here in the image.