How do you know if a new substance is formed in a chemical reaction? Please explain why​

For which one of the following reactions is the value of δh° rxn equal to δh° f for the product? a. 2 h2 (g) + o2 (g) → 2 h2o (l) b. n2 (g) + o2 (g) → 2 no (g) c. 2 h2 (g) + o2 (g) → 2 h2o (g) d. h2o (l) + 1/2 o2 (g) → h2o2 (l) e. none of the above

If the element whose electric configuration ends in the d sublevel, the element is calssified as? a. inner transition b. noble gases c. representative d. transition

The density of an unknown gas at 98°c and 740 mmhg is 2.50 g/l. what is the molar mass of the gas with work showed?