Bond length of ${\mathrm{H}}_2$ is $0.074nm$, Bond length of ${\mathrm{Cl}}_2$ is $1.98A°$. Bond length of HCl is

$$\begin{gathered} \text{ We are given: } \\ \text{ Bond length of hydrogen molecule }=0.074\mathrm{nm} \\ \text{ Bond length of chlorine molecule }=1.98\text{Å}=0.0198\mathrm{nm} \\ \text{ (Conversion factor: }1\mathrm{nm}=10\text{Å}) \\ \text{ To calculate the bond length of }\mathrm{HCl}\text{, we use the formula: } \\ {\mathrm{l}}_{\mathrm{HCl}}=\frac{{\mathrm{l}}_{{\mathrm{H}}_2}}{2}+\frac{{\mathrm{l}}_{{\mathrm{Cl}}_2}}{2} \\ \text{ Putting values in above equation, we get: } \\ \begin{array}{rl} & {\mathrm{l}}_{\mathrm{HCl}}=\frac{0.074}{2}+\frac{0.198}{2}\\ & {\mathrm{l}}_{\mathrm{HCl}}=0.136\mathrm{nm}\end{array} \\ \text{Hence, the bond length of}\mathrm{HCl}\text{is}0.136\mathrm{nm}\text{.} \end{gathered}$$