Question

Question #59501

Answer 1

Hydrogen (AMU=2) exists as `H_2`, so 1 gram equals `1/2` mole

Oxygen (AMU=16) exists as `O_2`, so 8 gram equals `1/2` mole

BUT!
Water = `H_2O`, so whereas all of the Hydrogen `H_2` will be used, only half of the Oxygen will be used.
Therefore, 1g `H_2` + 4g `O_2` `rarr` 5 g `H_2O`.....

So, questionwise, back to the Drawing Board....
(Or the CAD-terminal probably these days...)

For the rest, if you have 3g `H_2`, and clearly an abundance of Oxygen, these 3 grams will react with 3x4=12 grams of O2, resulting in 15 g `H_2O`.