13. State any one condition under which a bimolecular reaction may be kinetically of first order?

${\mathrm{CH}}_3-{\mathrm{COOC}}_2{\mathrm{H}}_5+\mathrm{HOH}\to {\mathrm{CH}}_3-\mathrm{COOH}+{\mathrm{C}}_2{\mathrm{H}}_5-\mathrm{OH}$
t = 0  0.01 mol 10 mol       0 mol              0 mol
t             0 mol 9.9 mol      0.01 mol    0.01 mol

The concentration of water does not get altered much during the course of the reaction. So, in the rate equation
$\mathrm{Rate} = \mathrm{k}[{\mathrm{CH}}_3.{\mathrm{COOC}}_2{\mathrm{H}}_5{]}^1 {\left[{\mathrm{H}}_2\mathrm{O}\right]}^0.$
the term [H2O] can be taken as constant. The equation, thus, becomes
Rate = k [CH₃COOC₂H₅]
where k = k′ [H2O]
and the reaction behaves as first order reaction