WAEC - Physics (2000 - No. 44)
The half life of a radioactive substance is 14 days. If 48g of this substance is stored, after how many days will 1.5g of the original substance remain?
84 days
70 days
56 days
40 days
Paliwanag
Using N = N\(_o\)\((\frac{1}{2})^n\)
N\(_o\) = 48g, N =1.5g and n = number of half-lives
1.5 = 48\((\frac{1}{2})^n\)
\(\frac{1.5}{48}\) = \(\frac{1}{32}\) = (\(\frac{1}{2})^5\) = \((\frac{1}{2})^n\)
n = 5 (same base, equate power)
Total time = n x t\(_{\frac{1}{2}}\) = 5 x 14 = 70 days
Therefore, 1.5g will remain after 70 days.