Question

One mole of anhydrous $Mg{Cl}_2$ dissolves in water and liberates $25\text{cal/mol}$ of heat. $\Delta H_{hydration}$ of $Mg{Cl}_2=-30\text{cal/mol}$. Heat of dissolution of $Mg{Cl}_2.H_{2}O$ is:

• A. $+5cal/mol$
• B. $-5cal/mol$
• C. $55cal/mol$
• D. $-55cal/mol$

Answer 1

Answer: (B)

$-5cal/mol$

$MgC{l}_2+H_{2}O\stackrel{\Delta H_{sol}}{\to }MgC{l}_2.H_{2}O$

$\Delta H_{sol}=-25/mol$ (given)

$MgC{l}_2+H_{2}O\stackrel{\Delta H_{sol}}{\to }M{g}_{\left(aq\right)}^{+2}+2C{l}_{\left(aq\right)}^{-}$

$\Delta H_{hydration}=-30cal/mol$

$\Delta H_{dissolution}=\Delta H_{hydration}-\Delta H_{solution}$

$=-30-(-25)$

$=-5cal/mol$

Hence, the correct option is $\text{B}$