Question

One mole of anhydrous salt $A B$ dissolves in water and liberates $21.0 \,J\, mol ^{-1}$ of heat. The value of $\Delta H_{\text {(hydration) }}$ of $A B$ is $-29.4\, J \,mol ^{-1} .$ The heat of dissolution of hydrated salt, $A B .2 H _{2} O _{(s)}$ is

• A. $50.4 \,J\, mol^{-1}$
• B. $8.4 \,J\, mol^{-1}$
• C. $-50.4 \,J\, mol^{-1}$
• D. $-8.4 \,J\, mol^{-1}$

Answer 1

Answer: (B)

$\Delta H_{\text {solution}}=\Delta H_{\text {dissolution}}+\Delta H_{\text {hydration}}$ $-21.0=\Delta H_{\text {dissolution}}+(-29.4)$

$\Delta H_{\text {dissolution}} =-21.0+29.4=8.4 J\, mol^{-1}$