Question

Find the number of moles of water that can be formed if you have 134 mol of hydrogen gas and 62 mol of oxygen gas

Answer 1

164. You have excess hydrogen- you need double the number of moles hydrogen as oxygen because water has two hydrogens and only one oxygen while both hydrogen gas and oxygen gas are diatomic. 

Because oxygen is diatomic, you spit it in half and basically double the number of moles, tack on your excess hydrogen, and you have 164 moles of water. 

Edit: Lets look at it in one more way, taking into account what a mole is: 

A mole is Avogadro's Constant number of particles- or about 6.02x10^23. So, we can multiply the number of moles you have to figure out how many particles of each we have. You can skip this step, because we will convert back to moles, but its illustrative of the concept. 

6.02x10^23 x 174 moles H2 = 1.05 x 10^26 particles H2 
6.02x10^23 x 82 moles O2 = 4.94 x 10^25 particles O2 

I'm going to take one more step here- take them out of diatomic form. This will double the number of particles of each. 

2.10 x 10^26 particles H+ 
9.88 x 10^25 particles O(2-) 

Now, we know we need a 2:1 oxygen atom to hydrogen atom ration, so we devide and see oxygen is our limiting reagent. So, we have 9.88 x 10^25 single oxygen atoms, all of which will become water the hydr