Chemical Thermodynamics Question 358
Question: At $ 25{}^\circ C $ , when 1 mole of $ MgSO_4 $ was dissolved in water, the heat evolved was found to be $ 91.2kJ. $ One mole of $ MgSO_4 $ . $ 7H_2O $ on dissolution gives a solution of the same composition accompanied by an absorption of $ 13.8kJ $ . The enthalpy of hydration, i.e., $ \Delta H_{h} $ for the reaction $ MgSO_4( s )+7H_2O( l )\xrightarrow{{}}MgSl_4.7H_2O( s ) $ is:
Options:
A) $ -105kJ/mol $
B) $ -77.4kJ/mol $
C) $ 105kJ/mol $
D) None of these
Show Answer
Answer:
Correct Answer: A
Solution:
- Given that $ MgSO_4(s)+nH_2O\to MgSO_4nH_2O; $
$ {\Delta_{r}}H_1=-91.2kJ/mol $ ….(i) $ MgSO_4.7H_2O(s)+(n-7)H_2O\to MgSO_4(nH_2O) $
$ {\Delta_{r}}H_2=13.8kJ/mol $ …(ii) or $ \Delta H_{hyd}={\Delta_{r}}H_1-{\Delta_{r}}H_2 $ equation (i) - (ii) $ =-91.2kJ/mol-13.8kJ/mol $
$ =-105kJ/mol $
