The enthalpies of combustion of formaldehyde and paraformaldehyde (a polymer of formaldehyde) are -134 and -732 kcal/mol respectively. The enthalpy of polymerisation per mole of paraformaldehyde is - 72 kcal The molecular formula of paraformaldehyde is :

$\mathrm{nHCHO}⟶(\mathrm{HCHO}{)}_{\mathrm{n}}$

Formaldehyde and paraformaldehyde are burned as follows:

$\left[\mathrm{HCHO}+{\mathrm{O}}_2\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\right]\times \mathrm{n}{\mathrm{\Delta }}_{\mathrm{c}}\mathrm{H}=-134\mathrm{kcal}/\mathrm{mol}$

$(\mathrm{HCHO}{)}_{\mathrm{n}}+{\mathrm{nO}}_2\to {\mathrm{nCO}}_2+{\mathrm{nH}}_2\mathrm{O}{\mathrm{\Delta }}_{\mathrm{c}}\mathrm{H}=-732\mathrm{kcal}/\mathrm{mol}$

To obtain the polymerization of formaldehyde, subtract equations (I) and (ii) as follows:

$\mathrm{n}\left(\mathrm{HCHO}\right)\to (\mathrm{HCHO}{)}_{\mathrm{n}}$
$$\begin{gathered} -72=-134\mathrm{n}+732 \\ \Rightarrow 134\mathrm{n}=804 \\ \Rightarrow \mathrm{n}=6 \end{gathered}$$