At 25 oC, hydrogen iodide breaks down very slowly to hydrogen gas and iodine vapor with a rate constant of 2.4 x 10-21L/mol. s. If 0.0100 mol of HI(g) is placed into a 1.0 L container at 25 oC, how long will it take for the concentration of HI to reach 0.00900 mol/L?

Explanation:

Whenever a question asks you, "What is the concentration after a given time?" or something like that, you must use the appropriate integrated rate law expression.

The reaction is 2nd order, because the units of k are L·mol⁻¹s⁻¹.

The integrated rate law for a second-order reaction is  

Data:

   k = 2.4 × 10⁻²¹ L·mol⁻¹s⁻¹

[A]₀ = 0.0100     mol·L⁻¹

[A] = 0.009 00 mol·L⁻¹

Calculation :

neutrons are in the middle of the atom and the electrons, which are much smaller.