Question

Question #aeef4

Answer 1

0.25g

Explanation:

The half life of a substance is how long it takes for any given mass of that substance to decay to half of its prior mass. The key word here is "prior" mass; not initial mass. After the first half life, half of the original mass is left, but after the second half life, half of the mass that was present after the first half life is gone.

Numerically, this would look as such. Provided that `x` is our initial mass, then after a period of time equal to one half life `h`...

`t=0, mass=x`
`t=h, mass=x/2`
`t=2h, mass=x/4`
`t=3h, mass=x/8`
`...`
`t=nh, mass = x/2^n`

The half life of the substance is 11.7 days. Dividing our time elapsed by this, we find that `35.1 = 3(11.7) = 3h`. Thus, using the formulae we found above...

`m_(t=3h) = x/8 = 2.0/8 g = 0.25g`