Explanation:
According to law of conservation of mass, mass of the reactants is equal to the mass of products in a chemical equation. As mass can neither be created nor it can be destroyed but it can be transformed from one form to another.
As it is given that hydrogen and in excess oxygen is reacting that leads to the formation of water. Hence, the chemical reaction equation will be as follows.

Since, it is given that 4 mol of hydrogen is reacting with excess of oxygen and gives 2 moles of water.
Hence, number of moles of water produced is calculated as follows.
4 mol of 
= 4 moles of 
Thus, we can conclude that 4 moles of water you can produce from 4.0 mol of hydrogen and excess oxygen.
Answer:

Explanation:
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In this case, since perchloric acid is HClO4 and is a strong acid and calcium hypochlorite is Ca(ClO)2, the undergoing molecular chemical reaction turns out:

Thus, since the resulting hypochlorous acid is weak, it does not fully ionize, so it remains unionized, however, we can write the ions for the other species:

Now, we can cancel out the spectator ions, calcium and perchlorate, to obtain:

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As the atomic radius decreases, it becomes harder to remove an electron that is closer to a more positively charged nucleus. ... They experience a weaker attraction to the positive charge of the nucleus. Ionization energy increases from left to right in a period and decreases from top to bottom in a group.
The pencil attached to the negative terminal of the battery collects hydrogen gas while the one connected to the positive terminal collects oxygen.
The balanced chemical equation is:
2H2 + O2 ---> 2H2O
We are given the amount of the product produced from the reaction. This will be the starting point for the calculations.
355 g H2O ( 1 mol H2O/ 18.02 g H2O) ( 1 mol O2 / 2 mol H2O ) ( 32 g O2 / 1 mol O2 ) = 315.205 g O2