Question

Combustion of glucose takes place as

\[\ce{C6H12O6_{(s)} + 6O2_{(g)} -> 6CO2_{(g)} + 6H2O_{(g)}}\]; ΔH = −72 kcal mol⁻¹

The energy needed for the production of 1.8 g of glucose by photosynthesis will be ___________.

Options

• 0.82 kcal

• 0.72 kcal

• 8.2 kcal

• 7.2 kcal

Answer 1

Combustion of glucose takes place as

\[\ce{C6H12O6_{(s)} + 6O2_{(g)} -> 6CO2_{(g)} + 6H2O_{(g)}}\]; ΔH = −72 kcal mol⁻¹

The energy needed for the production of 1.8 g of glucose by photosynthesis will be 0.72 kcal.

Explanation:

The equation for photosynthesis is the reverse of combustion of glucose.

∴ \[\ce{6CO2_{(g)} + 6H2O_{(g)} -> C6H12O6_{(s)} + 6O2_{(s)}}\]; ΔH = 72 kcal mol⁻¹

Molecular mass of glucose = 180 g mol⁻¹

180 g mol⁻¹ ≡ 72 kcal mol⁻¹

∴ 1.8 g ≡ `(72  "kcal mol"^-1 xx 1.8  "g")/(180  "g mol"^-1)`

= 0.72 kcal