$\underset{\underset{{{C}_{n}}{{H}_{2n+1}}COOH+NaOH+NaOH\to {{C}_{n}}{{H}_{2n+1}}COONa+{{H}_{2}}O}{\mathop{\downarrow }}\,}{\mathop{R}}\,COOH+NaOH\to RCOONa+{{H}_{2}}O$ $NaOH\left\{ \begin{matrix} V=200mL=0/2L  \\ pH=14\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-14}}\Rightarrow   \\ \end{matrix}\left[ O{{H}^{-}} \right]=M=1mol.{{L}^{-1}} \right\}\Rightarrow 0/2mol$ $0/2mol=56/8\times \frac{1mol}{14n+46}\Rightarrow n=17\Rightarrow $ در مجموع 18 کربن دارد