Question

A drug suspension decomposes by zero-order kinetics with a rate constant of 2 mg mL-1month-1. If the initial concentration is 100 mg mL-1, what is the shelf life (t10%)?

Answer 1

Answer: 5 months

Explanation: Rate Law for a reaction is an equation relating the reaction rate of reactants. It is expressed as:

aA + bB → cC + dD

$r=k[A]^{a}[B]^{b}$

in which k is the rate constant of the reaction

Zero-order Kinetics is a chemical reaction in which the rate doesn't change with the variations in concentration of the reactants. At this state, it is possible to determine the final concentration by using:

$[A]=-kt+[A_{0}]$

Shelf-Life is the time required for 10% of a drug or material to desintegrate: it is the time necessary for A to desintegrate 90% of its original concentration.

Then, the shelf-life for the drug is

$[A]=-kt+[A_{0}]$

$90=-2t+100$

2t = 10

t = 5

Shelf-life for a drug which decomposes by zero-order kinetics with rate constant of 3mgmL^{-1}month^{-1} is 5 months