[Single-digit Integer] The heat evolved on combustion of 1 g of starch, (C6H10O5)x, into (g) and (l) is 4.6 kcal. Heat of formation of (g) and (l) are -94.2 and -68.4 kcal/mol, respectively. The magni

Question

[Single-digit Integer] The heat evolved on combustion of 1 g of starch, (C6H10O5)x, into $CO_2$(g) and $H_2O$(l) is 4.6 kcal. Heat of formation of $CO_2$(g) and $H_2O$(l) are -94.2 and -68.4 kcal/mol, respectively. The magnitude of standard enthalpy of formation of 1 g of starch (in kcal) is

Prashant Kotian · Accepted Answer

Correct answer: 0001. Combustion of 1 mole of glucose/starch unit: (C6H10O5) + 6 → 6 + 5 .
Mass of starch monomer unit = 162 g/mol. Heat evolved per gram = 4.6 kcal.
Thus, heat evolved per monomer unit = 162 × 4.6 = 745.2 kcal/mol.
ΔHc = [6 × (-94.2) + 5 × (-68.4)] - ΔfH°(starch) = -745.2 kcal.
-565.2 - 342.0 - ΔfH°(starch) = -745.2 → ΔfH°(starch) = -162 kcal/mol.
Standard enthalpy of formation of 1 g of starch = -162 / 162 = -1 kcal.
Magnitude is 1.

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