Answer is: the oxidation number of oxygen must decrease.
Oxidizing agent is element (substance) that gain electrons in chemical reaction.
For example, balanced chemical reaction: C + O₂ → CO₂.
Oxygen change oxidation number from 0 to oxidation number -2 (reduction) and carbon change from 0 to +4 (oxidation)
The reaction is as follows:
2 H₂(g) + O₂(g) → 2 H₂O(g) . ΔH = - 483.5 kJ
Using the change in enthalpy and heat, calculate the moles as follows:
Moles of H₂ = -

x 2 mol H₂
= - 216 kJ / (-483.5 kJ) x 2 mol H₂
= 0.893 mol H₂
Calculate the mass of H₂ using the moles and molar mass as follows:
0.893 mol H₂ x (2.02 g H₂ / 1 mol H₂) = 1.79 g H₂
Therefore, the mass of hydrogen gas is 1.79 g
4-nonanone
Explanation: there is a functional group in the compound CH3CH2CH2CH2CH2COCH2CH2CH3 which is a ketone; So for naming a ketone we must first mention the name of the prefix that is correspond then add the suffix ''one''. And why I putted a ''4'' before the name is because we must indicate where this functional group is located, for doing that we have to start counting where the functional group is the closet to the end of the compound so when looking we should start to count on the right side. this is how I get 4-nonanone