The rate of reaction, $\mathrm{A}+\mathrm{B}\to \mathrm{products} ,$is given by the reaction, $\mathrm{r}=\mathrm{k}\left[\mathrm{A}\right]\left[\mathrm{B}\right].$ If B is taken in large excess the order of reaction would be

The reaction is $\mathrm{A}+\mathrm{B}\to \mathrm{products}$

Rate equation is $\mathrm{r}=\mathrm{K}\left[\mathrm{A}\right]\left[\mathrm{B}\right]$

According to chemical kinetics, in a reaction if one of the reactant is taken in excess amount then there is no change of its concentration in a reaction. Hence, the order is not consider to that reactant. Here, for a given reaction amount of B is in large excess. Therefore, order of B is taken as zero order. The order with respect to A is 1.