NEET 2025 — Chemistry solution

NEET2025Chemistry

The standard heat of formation, in kcal/mol, of Ba $^{2 +}$ is:

[Given: standard heat of formation of SO $_{4}^{2 -}$ ion (aq) $= - 216$ kcal/mol, standard heat of crystallisation of BaSO $_{4}$ (s) $= - 4.5$ kcal/mol, standard heat of formation of BaSO $_{4}$ (s) $= - 349$ kcal/mol]

A
$-$ 128.5
B
$-$ 133.0
C
+133.0
D
+220.5

Solution

For the crystallisation process: Ba $^{2 +}$ (aq) + SO $_{4}^{2 -}$ (aq) $\to$ BaSO $_{4}$ (s), $Δ H_{cr y s} = - 4.5$ kcal/mol.

By Hess's law: $Δ H_{cr y s} = Δ H_{f} (BaSO_{4}) - [Δ H_{f} (Ba^{2 +}) + Δ H_{f} (SO_{4}^{2 -})]$ .

$- 4.5 = - 349 - [Δ H_{f} (Ba^{2 +}) + (- 216)] = - 349 - Δ H_{f} (Ba^{2 +}) + 216 = - 133 - Δ H_{f} (Ba^{2 +})$ .

$Δ H_{f} (Ba^{2 +}) = - 133 - (- 4.5) = - 128.5$ kcal/mol.

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