Answer:
15 moles
Explanation:
Data given:
mass of hydrogen (H₂) = 30.0 g
amount of oxygen (O₂) = excess
moles of water = ?
Solution:
First we look to the reaction in which hydrogen react with oxygen and make (H₂O) 
Reaction: 
               2H₂  + O₂  -----------> 2H₂O
Now look at the reaction for mole ratio
              2H₂  + O₂  -----------> 2H₂O
              2 mole                       2 mole
So it is 2:2 mole ratio of hydrogen to water
As we Know
molar mass of H₂  = 2(1) = 2 g/mol
molar mass of H₂O = 2(1) + 16 = 18 g/mol
Now convert moles to gram
                   2H₂         +       O₂        ----------->    2H₂O
           2 mole (2 g/mol)                                 2 mole (18 g/mol)
                     4 g                                                     36 g
So, 
we come to know that 4 g of hydrogen gives 36 g of water then how many grams of water will be produce by 30 grams of hydrogen.
Apply unity formula
                         4 g of H₂ ≅ 36 g of H₂O 
                         30 g of H₂ ≅ X of H₂O 
Do cross multiplication
                   X of H₂O =  30 g x 36 g / 4 g
                   X of H₂O =  270 g
Now convert grams of H₂O into moles
                No. of moles = mass in grams/molar mass
Put values in above formula
                No. of moles = 270 g / 18 (g/mol)
                No. of moles = 15 mol
so 30 gram of hydrogen produce 15 mol of water.