b
\(M=98\quad \quad \quad \quad \quad\quad \quad \quad \quad \quad M=135\)

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

(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 \(=2.70 \times \frac{90}{100} \,g\)

\(=2.43\, g\)

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

\(\Rightarrow x =243\)