**Problem.**
During the chemical reaction

*2 mol* of oxygen

*O₂*
interacting with

*4 mol* of hydrogen

*H₂* produces

*4 mol* of water

*H₂O*
2H₂ + O₂ → 2H₂O

Heat released during the process is equal to

*9.7×10⁵ J*.
What fraction of mass is transformed into heat during reaction?

**Solution.**
Mass and energy are related by the equation

E = mc²

and thus energy released during chemical reaction is

δE = δmc²

where

*δm* is change in the mass of interacting particles.
So

δm = = = 1.08×10⁻¹¹ kg

Molar mass of hydrogen

*H₂* is equal to

*2 g*,
while molar mass of oxygen is

*32 g* so original mass of reactants is

m = 7.2×10⁻² kg

and fraction of mass transformed into heat during reaction is

= = 1.5×10⁻¹⁰ kg