$C _{6} H _{5} NH _{2}$ $\xrightarrow{{90\,\% \,efficiency}}$ $\begin{array}{*{20}{c}} {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} \\ {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\ {{C_6}{H_5} - NH - C - C{H_3}} \end{array}$

(Aniline)$\quad \quad \quad \quad \quad\quad \quad \quad$ (Acetanilide)

Given $1.86 \,g$

$\Rightarrow$ $1\, mol$ $C _{6} H _{5} NH _{2}$ give $1\, mol$ $\begin{array}{*{20}{c}} {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} \\ {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\ {{C_6}{H_5} - NH - C - C{H_3}} \end{array}$

$\therefore$ moles of $C _{6} H _{5} NH _{2}=$ moles of $C _{6} H _{5} NH _{2}$ give $1\, mol$ $\begin{array}{*{20}{c}} {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} \\ {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\ {{C_6}{H_5} - NH - C - C{H_3}} \end{array}$

$\Rightarrow \frac{1.86}{93}=\frac{ W _{\text {acetanilide }}}{135}$

$\Rightarrow W_{\text {acelanilide }}=\frac{1.86 \times 135}{93}\, g =2.70\, g$

But efficiency of reaction is $90\, \%$ only

$\therefore$ Mass of acetanilide produced $=2.70 \times \frac{90}{100} \,g$

$=2.43\, g$

$=243 \times 10^{-2}\, g$

$\Rightarrow x =243$