Question

One mole of soda ash dissolves in water with evolution of 25 kJ of heat. Dissociation of 1 mol of the hydrate crystals in water resulted in the absorption of 67 kJ of heat. The heat released due to hydration of soda ash is

• A. $-92kJ$
• B. $+42kJ$
• C. $-42kJ$
• D. $-102kJ$

Answer 1

The correct option is A $-92kJ$

From Hess's law, We know
$△H_{soln}\left(anhydroussalt\right)=△H_{hydr.}+△H_{soln}\left(Hydratedsalt\right)$

$\Rightarrow △H_{hydr.}=△H_{soln}\left(anhydroussalt\right)-△H_{soln}\left(Hydratedsalt\right)$
Given,
$△H_{soln}\left(Hydratedsalt\right)=-25kJ△H_{soln}\left(anhydroussalt\right)=+67kJ△H_{hydr.}=Tobecalculated$
Putting all values in the formula, we get;
$△H_{hydr.}=-25-(+67)$
$\Rightarrow △H_{hydr.}=-92kJ$