A reaction in which one reactant is present in large amount and its concentration does not change during the course of the reaction and it behaves as first order reaction is called pseudo first order reaction.
e.g., (i) hydrolysis of ethyl acetate
Rate of reaction $=k^{\prime}\left[\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5\right]\left[\mathrm{H}_2 \mathrm{O}\right]$
where $k=k^{\prime}\left[\mathrm{H}_2 \mathrm{O}\right]$
e.g., (ii) inversion of cane sugar
$\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+\mathrm{H}_2 \mathrm{O} \stackrel{\mathrm{H}^{+}}{\longrightarrow} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$
Rate of reaction $=k^{\prime}\left[\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right]\left[\mathrm{H}_2 \mathrm{O}\right]$
$=k\left[\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right]$
where $k=k^{\prime}\left[\mathrm{H}_2 \mathrm{O}\right]$