1.

Calculate the enthalpy of vaporisation for water form the following: `H_(2)(g) +1//2O_(2)(g) rarr H_(2)O(g),DeltaH =- 57.0 kcal` `H_(2)(g) +1//2O_(2)(g) rarr H_(2)O(l),DeltaH =- 68.3 kcal` Also calculate the heat required to change `1g H_(2)O(l)` to `H_(2)O(g)`

Answer» Given
`{:(H_(2)(g)+1//2O_(2)(g)toH_(2)O(g), Delta=-H =- 94.3 kcal ...(i)),(H_(2)(g)+1//2O_(2)(g)toH_(2)O(l),DeltaH =68.3 kcal..(ii)),(" - - - + "),(ulbar{:("Subtracting equation (ii)from equation(i),"),(H_(2)O(l)toH_(2)O(g), DeltaH =+11.3 kcal):}):} `
Therefore, Enthalpy of vaporisation for `H_(2)O = +11.3 kcal`
Also `18g H_(2)O` requires enthalpy of vaporisation `=+11.3 kcal`
`:. 1 g H_(2)O` required enthalpy of vaporisation `= 11.3//8 kcal = 0.628 kcal`


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