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Question

The free energy change when 1 mole of NaCl is dissolved in water at 298 K is x kJ. Find the value of 'x'.
Given:-
Lattice energy of NaCl=778 kJ mol1
Hydration energy of NaCl=775 kJ mol1
Entropy change (at 298 K) =40J mol1

A
x = 9
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B
x = -9
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C
x = 9000
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D
x = -9000
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Solution

The correct option is A x = 9
For the dissolution of a compound, we have the following relation
ΔHdissolution=ΔH(lattice energy)+ΔH(hydration)

ΔH(lattice energy)=778 kJΔH(hydration)=775 kJ

Putting the above values, we get;
ΔHdissolution=778775=3 kJ mol1=3000J mol1

Given,
ΔSdissolution=40 J mol1

Also we know,
G=ΔHTΔS=3000300×40=9000J
ΔG=9 kJ=x kJ x=9

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