ThermochemistryHard

Question

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

Options

A.CH2O
B.C6H12O6
C.C3H6O3
D.C4H8O4

Solution

$nHCHO(g) \rightarrow (HCHO)_{2}$

$\Delta H = - 72 = n \times ( - 134) - ( - 732) \Rightarrow n = 6$

∴ Molecular formula = $(HCHO)_{6} = C_{6}H_{12}O_{6}$

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