Question

Potassium-40 has a half-life of 1.3 billion years. as the potassium-40 isotope decays, it becomes argon. if a rock was formed with 12 g of potassium-40, approximately how long would it take for 75% of the potassium-40 to be replaced by argon?
a. 1.3 billion years
b. 2.6 billion years
c. 5.2 billion years
d. 650 million years

Answer 1

The half life of a substance is the time taken by a radioactive substance to decay by half its original mass. In this case, the half life of Potassium-40 is 1.3 billion years. 
Original mass of Potassium-40 = 12 g (100%)
New mass after the decay = 3 g ( 25 %, since 75% was replaced by argon)
New mass = Original mass × (1/2)^n ; where n is the number of half lives. 
 3 = 12 × (1/2)^n
(1/2)^n = 1/4 
      n = 2 
Therefore; the time taken will be 1.3 × 2 = 2.6 Billion years

Answer 2

Answer:Potassium is a crucial element for the healthy operation of the human body. Potassium occurs naturally in our environment (and thus our bodies) as three isotopes: Potassium-39, Potassium-40, and Potassium-41. Their current abundances are 93.26%, 0.012% and 6.728%. A typical human body contains about 3.0 grams of Potassium per kilogram of body mass.

How much Potassium-40 is present in a person with a mass of 80 kg?

If, on average, the decay of Potassium-40 results in 1.10 MeV of energy absorbed, determine the effective dose (in Sieverts) per year due to Potassium-40 in an 80-kg body. Assume an RBE of 1.2. The half-life of Potassium-40 is 1.28 x 109 years.

Explanation: