Introduction:
When hydrogen and oxygen combine to form water, the ratio of hydrogen to oxygen by mass is 1:8. In this explanation, we will calculate the number of moles of water that will be formed when 10 moles of hydrogen gas and 5 moles of oxygen gas react.

Given:
- Hydrogen gas (H2): 10 moles
- Oxygen gas (O2): 5 moles
- Ratio of hydrogen to oxygen by mass: 1:8

Calculating the moles of oxygen:
Since oxygen gas (O2) is diatomic, we need to divide the given number of moles by 2 to get the moles of oxygen atoms. Therefore, the moles of oxygen atoms will be 5/2 = 2.5 moles.

Calculating the moles of hydrogen:
The given moles of hydrogen gas (H2) directly represent the moles of hydrogen atoms. Therefore, the moles of hydrogen atoms will be 10 moles.

Determining the limiting reactant:
To find the limiting reactant, we compare the moles of each element to the stoichiometric ratio of the reaction. The stoichiometric ratio tells us the mole-to-mole ratio of reactants required for the reaction.

In this case, the stoichiometric ratio is 1:8. Therefore, for every 1 mole of hydrogen, we need 8 moles of oxygen. Since we have 10 moles of hydrogen and 2.5 moles of oxygen, we can see that the oxygen is the limiting reactant.

Calculating the moles of water formed:
Since oxygen is the limiting reactant, we use its moles to determine the moles of water formed. According to the stoichiometric ratio, for every 8 moles of oxygen, 1 mole of water is formed.

Using the given 2.5 moles of oxygen, we can calculate the moles of water formed using the following proportion:

(2.5 moles O2) x (1 mole H2O / 8 moles O2) = 0.3125 moles H2O

Therefore, when 10 moles of hydrogen gas and 5 moles of oxygen gas react, the number of moles of water formed is approximately 0.3125 moles.