When `12.0g` of `C` reacted with oxygen to form `CO` and `CO_(2)` at `25^(@)C` at constant pressure, `313.8 kJ` of heat was released and no carbon remained. Calculate the mass of oxygen which reacted.
`Delta_(f)H^(Theta) (CO,g) =- 110.5 kJ mol^(-1)` and
`Delta_(r)H^(Theta) (CO,g) =- 393.5 kJ mol^(-1)`

Explanation

`{:(C + (1)/(2)O_(2) rarr CO ,,DeltaH =- 110.5),(C+O_(2)rarrCO_(2),,DeltaH =- 393.5):}`
Let `x mol` of `C` is converted into `CO` and `(1-x)` mol into `CO_(2)`.
`:.x (-110.5) +(1-x)(-393.5) =- 313.8`
`x = 0.2816 mol`
Amount of `O_(2)` needed `= (0.2816)/(2) +(1-0.2816)`
`= 0.8592 mol = 0.8592 xx 32`
`= 27.5 g`