The half-life of a radioactive substance is 14 days. After 10 days a sample of the substance has been reduced to a mass of 4 mg. What was the initial mass (in mg) of the sample?

Answer 1

A = A₀ e^kt

When A = 1/2 A_0, t is 14 days

1/2 = e^14k

ln (1/2) = 14k ln(e)

k = ln(1/2) / 14 = -0.0495

When t = 10 days, A = 4mg

4 = A₀ e^{(-0.0495 *10)}

Solve using calc

A₀ = 6.6mg