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Heat of Formation

The standard ...

Question

The standard heat of formation of sodium ions in aqueous solution from the following data:
Heat of formation of $N a O H (a q .)$ at $25^{o} C = - 470.7 k J$
Heat of formation of ${O H}^{-} (a q .)$ at $25^{o} C = - 228.8 k J$ is:

A

- 251.9 k J

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B

241.9 k J

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C

- 241.9 k J

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D

251.9 k J

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Solution

The correct option is D $- 241.9 k J$
Solution:- (C) $- 241.9 K J$
Given:-
${Δ H_{f}}_{N a O H} = - 470.7 K J$
${Δ H_{f}}_{{O H}^{-}} = - 228.8 K J$
${Δ H_{f}}_{H^{+}} = ?$
$N a O H ⟶ {N a}^{+} + {O H}^{-}$
${Δ H_{f}}_{N a O H} = {Δ H_{f}}_{{O H}^{-}} + {Δ H_{f}}_{H^{+}}$
$\Rightarrow - 470.7 = - 228.8 + {Δ H_{f}}_{H^{+}}$
$\Rightarrow {Δ H_{f}}_{H^{+}} = - 470.7 + 228.8 = - 241.9 K J$
Hence the required answer is $- 241.9 K J$ .

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