The rate constant of a zero-order reaction is 2 × 10−2 mol L−1 s−1. If the concentration of the reactant after 25 s is 0.5 M, what was its initial concentration?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1.0 M
In a zero-order reaction the rate does not depend on concentration, so the reactant is used up at a constant rate k. The integrated rate equation is:
\( \displaystyle [R]=[R]_0-kt \qquad\text{so}\qquad [R]_0=[R]+kt \)
Substituting [R] = 0.5 M, k = 2 × 10−2 mol L−1 s−1 and t = 25 s:
\( \displaystyle [R]_0=0.5\,\mathrm{M}+(2\times10^{-2}\,\mathrm{mol\,L^{-1}\,s^{-1}})(25\,\mathrm{s}) \)
\( \displaystyle [R]_0=0.5\,\mathrm{M}+0.5\,\mathrm{M}=1.0\,\mathrm{M} \)