A reaction in which one reactant is present in large amount and its concentration does not get altered during the course of the reaction, behaves as first order reaction. Such reaction is called pseudo first order reaction.

e.g., (i) hydrolysis of ethyl acetate

$$\text { Rate of reaction }=k\left[\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5\right]$$

where, $k=k^{\prime}\left[\mathrm{H}_2 \mathrm{O}\right]$

e.g., (ii) inversion of cane sugar

$$\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+\mathrm{H}_2 \mathrm{O} \xrightarrow{\mathrm{H}^{+}} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$$

Rate of reaction $=k\left[\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right]$

where $\quad k=k^{\prime}\left[\mathrm{H}_2 \mathrm{O}\right]$