In order to decompose 9g water 142.5 kJ heat is required. Hence the enthalpy of formation of water is

Options

A. `-285kJ`
B. `+285kJ`
C. `-142.5kJ`
D. `+142.5kJ`

Answer

C

Explanation

`Delta H = Sigma H_(("product"))^(0) - SigmaH_(("reactant"))^(0)`
`2H_(2)O rarr 2H_(2) + O_(2)`
`Delta H_(f) = 2 xx H_((O_(2)))^(0) -2 xx H_((H_(2)O))^(0)`
`=0 + 0-2 xx 142.5 = -285kJ`
`Delta H_(f)` for 1 mole of `H_(2)O = -142.5kJ`