Radioactive decay. Carbon-14 has a decay rate of 0.012097% per year. The rate of change of an amount N of carbon-14 is given by

where t is the number of years since decay began.

a) Let N0 represent the amount of carbon-14 present at t = 0. Find the exponential function that models the situation.

b) Suppose 200 g of carbon-14 is present at t = 0. How much will remain after 800 yr?

c) After how many years will half of the 200 g of carbon-14 remain?

Respuesta :

a) The exponential function that models the situation is [tex]n(t) = n_{o}\cdot e^{-1.2097\times 10^{-4}\cdot t}[/tex].

b) The mass of Carbon-14 after 800 years is approximately 181.552 grams.

c) Half of the 200 g-Carbon 14 will remain after approximately 5729.91 years.

a) Mass of radioactive isotopes decay exponentially in time. The decay model is described below:

[tex]n(t) = n_{o}\cdot e^{-k\cdot t}[/tex] (1)

Where:

  • [tex]n_{o}[/tex] - Initial amount of Carbon-14, in grams.
  • [tex]n(t)[/tex] - Current amount of Carbon-14, in grams.
  • [tex]k[/tex] - Decay rate, no units.
  • [tex]t[/tex] - Time, in years.

If we know that [tex]k = 1.2097\times 10^{-4}[/tex], then the exponential function that models the situation is:

[tex]n(t) = n_{o}\cdot e^{-1.2097\times 10^{-4}\cdot t}[/tex] (1)

The exponential function that models the situation is [tex]n(t) = n_{o}\cdot e^{-1.2097\times 10^{-4}\cdot t}[/tex].

b) We determine the current amount of Carbon-14 by simply replacing all known variables: ([tex]n_{o} = 200\,g[/tex] and [tex]t = 800\,yr[/tex])

[tex]n(800) = 200\cdot e^{-1.2097\times 10^{-4}\cdot 800}[/tex]

[tex]n(800) \approx 181.552\,g[/tex]

The mass of Carbon-14 after 800 years is approximately 181.552 grams.

c) If we know that [tex]n = 100\,g[/tex] and [tex]n_{o} = 200\,g[/tex], then the time passed is:

[tex]100 = 200\cdot e^{-1.2097\times 10^{-4}\cdot t}[/tex]

[tex]0.5 = e^{-1.2097\times 10^{-4}\cdot t}[/tex]

[tex]t \approx 5729.91\,yr[/tex]

Half of the 200 g-Carbon 14 will remain after approximately 5729.91 years.

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