In a combustion experiment, it was found that 12.096 g of hydrogen molecules combined with 96.000 g of oxygen molecules to form water and released 1.715 × 103 kj of heat. calculate the corresponding mass change in this process. (1 j = 1 kg · m2/s2)

The mass change, or the mass defect, can be calculated by the formula that is very known to be associated with Albert Einstein. 

E = Δmc²
where
E is the energy gained or released during the reaction
c is the speed of light equal to 3×10⁸ m/s
Δm is the mass change

(1.715×10³ kJ)(1,000 J/1 kJ) = Δm(3×10⁸ m/s)²
Δm = 1.91×10⁻¹¹ kg

And a) oxygen i believe