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The standard enthalpy of formation of H(...

The standard enthalpy of formation of `H_(2)O(l)` and `H_(2)O(g)` are -285.8 and `-241.8 kJ "mol"^(-1)` respectively. What is the heat of vaporisation of water per gram at `25^(@)C` and 1 atm?

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For the vaporisation :
`H_(2)O(l) to H_(2)O(g)`
`Delta_(r)H^(@) = Delta_(f)H^(@)[H_(2)O(g)]-Delta_(f)H^(@)[H_(2)O(l)]`
`= -241.8 - (-285.8)`
`= 44 kJ "mol"^(-1)`
`therefore` Heat of vaporisation per gram = `44/18 = 2.44 kJ`
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