Why HCN is.more acidic then H20..
18. The correct- order of acidic strength is  

$$\begin{gathered} \left(1\right) {\mathrm{CH}}_3\mathrm{COOH} < \mathrm{HCN} < {\mathrm{H}}_2\mathrm{O} < {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \\ \left(2\right) {\mathrm{CH}}_3\mathrm{COOH} > \mathrm{HCN} > {\mathrm{H}}_2\mathrm{O} >{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \\ \left(3\right) \mathrm{HCN} > {\mathrm{CH}}_3\mathrm{COOH}>{\mathrm{H}}_2\mathrm{O} >{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \\ \left(4\right) {\mathrm{CH}}_3\mathrm{COOH} >\mathrm{HCN} >{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}>{\mathrm{H}}_2\mathrm{O} \end{gathered}$$