The unit of the rate constant ($k$) depends on the order of the reaction ($n$). The general expression for the unit is $(\text{mol L}^{-1})^{1-n}\text{s}^{-1}$ . For a zero-order reaction, the order $n = 0$. Substituting this into the general expression: $\text{Unit} = (\text{mol L}^{-1})^{1-0}\text{s}^{-1} = \text{mol L}^{-1}\text{s}^{-1}$ This unit is the same as the unit for the rate of reaction, which is consistent with the rate law for a zero-order reaction: $\text{Rate} = k[R]^0 = k$ .