Question

The initial concentration of Compound A is 1.5 mol/L. It disappears in a zero-order reaction with a rate constant of `"0.2 mol·L"^"-1""s"^"-1"`. What is its concentration after 15 s?

Answer 1

You use the integrated rate law for a zero order reaction:

[A] = [A]₀ - kt.

Insert the values of [A]₀, k and t into the equation and solve for [A].

You will get a value of -`1.5` mol/L.

It is impossible to have a negative concentration. This tells you that all the A has disappeared before 15 s.

If you set [A] = 0 and solve fot t, you get

`t = [A₀]/k = (1.5 " mol•L⁻¹")/(0.2 " mol•L⁻¹s⁻¹")` = 8 s

All the A has vanished after 8 s, so there is none after 15 s.