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NEET Chemistry Thermodynamics / रासायनिक उष्मागतिकी Question Bank Self Evaluation Test - Thermodynamics

  • question_answer
    At \[25{}^\circ C\], when 1 mole of \[MgS{{O}_{4}}\] was dissolved in water, the heat evolved was found to be \[91.2\text{ }kJ.\] One mole of \[MgS{{O}_{4}}\]. \[7{{H}_{2}}O\] on dissolution gives a solution of the same composition accompanied by an absorption of \[13.8\text{ }kJ\]. The enthalpy of hydration, i.e., \[\Delta {{H}_{h}}\] for the reaction \[MgS{{O}_{4}}\left( s \right)+7{{H}_{2}}O\left( l \right)\xrightarrow{{}}MgS{{l}_{4}}.7{{H}_{2}}O\left( s \right)\] is:

    A) \[-105\text{ }kJ/mol\]     

    B) \[-77.4\text{ }kJ/mol\]

    C) \[105\text{ }kJ/mol\]     

    D) None of these

    Correct Answer: A

    Solution :

    [a] Given that \[MgS{{O}_{4}}(s)+n{{H}_{2}}O\to MgS{{O}_{4}}n{{H}_{2}}O;\] \[{{\Delta }_{r}}{{H}_{1}}=-91.2kJ/mol\]          ....(i) \[MgS{{O}_{4}}.7{{H}_{2}}O(s)+(n-7){{H}_{2}}O\to MgS{{O}_{4}}(n{{H}_{2}}O)\] \[{{\Delta }_{r}}{{H}_{2}}=13.8kJ/mol\]            ...(ii) or \[\Delta {{H}_{hyd}}={{\Delta }_{r}}{{H}_{1}}-{{\Delta }_{r}}{{H}_{2}}\] equation (i) - (ii) \[=-91.2\text{ }kJ/mol-13.8\text{ }kJ/mol\] \[=-105kJ/mol\]


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