Why is this hydrogen a proton

Used the balanced equation 2h2+ o2 - -> 2h2o. if you have 7.2 grams of o2 , how many grams of h2o can you produce ?

Find the number of grams of hcl needed to react completely with .50 moles of magnesium.

How many liters of oxygen gas, at standard temperature and pressure, will react with 35.8 grams of iron metal? 4 fe (s) + 3 o₂ (g) → 2 fe₂o₃ (s)