Question

Concerning the reaction order, all of the following options are correct except which of the following?

• A. If doubling the concentration of a reactant doubles the rate of the reaction, then the reaction is first order in that reactant
  
• B. After three half-lives, a radioactive sample will have one-ninth of its original concentration
• C. The units for the rate constant for first-order reactions are $s^{-1}$
• D. Half life is independent of the concentration.

Answer 1

The correct option is B After three half-lives, a radioactive sample will have one-ninth of its original concentration

An isotope that has a half life of t minutes to decay to 1/8th of its original amount.

Note: $N=\frac{N_o}{2^n}=\frac{N_o}{2^3}=\frac{N_o}{8}$

Half-life ($t_{1/2}$) is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period.

So, the correct option is B