A drug becomes ineffective after `30%` decompoistion. The original concentration of a sample was `5 mg mL^(-1)` which becomes `4.2 mg mL^(-1)` during `20` months. Assuming the decompoistion of first order, calculate the expiry time of the drug in month. What is the half life of the Product?

Explanation

`k = (2.303)/(t) log.((a)/(a-x)) = (2.303)/(20) log.((5)/(4.2))`
`= 0.00872 month^(1-)`
expiry time `t` may be calculated as:
`k = (2.303)/(t) log.(a)/(a-x)`
`0.00872 = (2.303)/(t) log.(100)/(70) rArr t= 40.9 ~~ 41 months`
`t_(1//2) = (0.693)/(0.00872) = 79.4 months`