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Answer 1 · 2016-10-06T13:18:41

It is a fact (i) that most elemental gases are DIATOMIC; it is a fact (ii) that mass is always conserved in a chemical reaction.

Explanation:

Most of the elemental gases, $O_{2}$ $O_2$ , $N_{2}$ $N_2$ , $F_{2}$ $F_2$ , $C l_{2}$ $Cl_2$ are diatomic under standard conditions. The exceptions are the Noble Gases, which rarely form compounds. You simply have to know the so-called molecularity of oxygen, nitrogen, etc. Most inorganic chemists refer to these gases as $dinitrogen$ $"dinitrogen"$ , $dioxygen$ $"dioxygen"$ , $dihydrogen$ $"dihydrogen"$ to avoid ambiguity.

Knowledge of the molecularity of the elemental gases would be assumed for an A level student, and it certainly would be expected of a 1st year college student.

Now chemical reactions follow experimental result; experimental result does not follow our depiction of the equation. For the reaction of oxygen and hydrogen, we know that 1/2 an equiv of dioxygen reacts with one equiv of dihydrogen under normal conditions to give one equiv of water:

$H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l)$ $H_2(g) + 1/2O_2(g) rarr H_2O(l)$ $(i)$ $(i)$

If I wanted to, I could double the entire equation to get rid of the half-integral coefficients.

$2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)$ $2H_2(g) + O_2(g) rarr 2H_2O(l)$ $(i i)$ $(ii)$

The point is that these equations are representations of chemical reality. Now these reactions certainly conserve mass: in $(i)$ $(i)$ 18 g of dioxygen and dihydrogen combine to give 18 g of water (we would assume it to be a liquid under standard conditions!).

Given certain conditions, however, we could in fact produce hydrogen peroxide under so-called reducing conditions:

$H_{2} (g) + O_{2} (g) \to H O - O H (l)$ $H_2(g) + O_2(g) rarr HO-OH(l)$ $(i i i)$ $(iii)$

This would be admittedly hard to do, nevertheless, we can still conceive of the reaction as a formality, and we can certainly estimate the thermodynamic parameters involved in the reaction. In the reaction $(i i i)$ $(iii)$ , less energy would be evolved in the reaction because we are not completely reducing the oxygen gas, and we are only forming $2 \times H - O$ $2xxH-O$ bonds rather than the $4 \times H - O$ $4xxH-O$ bonds we form in $(i i)$ $(ii)$ . For another question dealing with the conservation of mass, see [here.](https://socratic.org/questions/what-is-a-balanced-chemical-equation-how-would-you-use-the-law-of-conservation-o)

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