Calculate the internal energy of combustion of D-ribose in Kj/mol. | TestHub

Passage / Comprehension

A sample of the sugar D-ribose (C₅H₁₀O₅) of mass $0.727 g$ was placed in a calorimeter and then ignited in the presence of excess oxygen. The temperature rose by $0.910 K$ . In a separate experiment in the same calorimeter, the combustion of $0.825 g$ of benzoic acid, for which the internal energy of combustion is $- 3251 k J m o l^{- 1}$ , gave a temperature rise of $1.940 K$ .

Given:

$Δ_{f} H_{C O_{2}}^{0} = - 393 k J m o l$

$Δ_{f} H_{H_{2} O}^{0} = - 286 k J m o l$

Calculate the internal energy of combustion of D-ribose in Kj/mol.

Options:

Answer:

Solution:

First, calibrate the calorimeter using benzoic acid:

Heat released by benzoic acid:

moles = $0.825/122 \approx 0.00676 m o l$

$q = 0.00676 \times (- 3251) \approx - 21.97 k J$

Temperature rise = $1.940 K$

So, the calorimeter constant is:

$C = 21.97/1.940 \approx 11.32 k J K^{- 1}$

Now for ribose:

$q = C \times Δ T = 11.32 \times 0.910 \approx 10.30 k J (re l e a se d)$

Moles of ribose = $0.727/150 \approx 0.00485 m o l$

$Δ U = - 10.30/0.00485 \approx - 2127 k J m o l^{- 1}$

Answer: (D) $- 2127.23 k J m o l^{- 1}$

Chemistry - Thermochemistry Question with Solution | TestHub

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