2H\(_2\) + O\(_2\) → 2H\(_2\)O  ΔH = -571kJ

For Hydrogen, Number of moles = \(\frac{mass}{Molar mass}\)

Stoichiometrically, 2  = \(\frac{mass}{2}\)

 Mass of Hydrogen = 2 x 2 = 4g
 If 4.0g of Hydrogen burns in O\(_2\) to produce - 571 kJ of energy
∴ 12.0g of Hydrogen will burn in O\(_2\) to produce  x kJ of energy
  x  =  \(\frac{{12}\times{- 571}}{4}\)

 =  - 1713kJ