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

Explanation

`underset"18 (g)"(H_2O) to H_2 + 1//2O_2`
9g `H_2O` is decomposed by -142.5 kJ amount of heat .
`therefore` 18 g of `H_2O` will be decomposed by `142.5/cancel9 xxcancel18^(2)`=+285 kJ
`therefore` 18 g of `H_2O` is formed by -285 kJ amount of heat (Evolution of heat =- ve sign)