If you react 2.00 g of hydrogen completely using 15.87 g of oxygen to produce water, how much water (in grams) will you have? (Please help, thank you!)

Answer 1

Hi Ella!

This question requires us to remember a couple of things about gases, but like all chemistry problems, we have to start with a balanced chemical equation.

We're given hydrogen (which is diatomic - H₂) reacting with oxygen (also diatomic O₂) to make H₂O.

This gives us the balanced chemical equation of...

2 H₂ + O₂ → 2H₂O

Since they've given us two quantities of starting materials, we know that this must be a limiting reactant problem...meaning we'll have excess of one of the reagents. I think the easiest way is to work all the way to the final amounts, and then just choose the one that produces less :)

Let's start by calculating the amount of water created based on the hydrogen!

2.00g H₂ * (1 mol H₂ / 2.02g H₂) * (2 mol H₂O / 2 mol H₂) * (18.02 g H₂O / 1 mol H₂O) = g H₂O

When we run those calculations out, based on the hydrogen alone, we can make 17.84g H₂O.

Now let's calculate the amount of water created based on oxygen...

15.87g O₂ * (1 mol O₂ / 32.0g O₂) * (2 mol H₂O / 1 mol O₂) * (18.02 g H₂O / 1 mol H₂O) = g H₂O

When we run those calculations out, based on the oxygen alone, we can make 17.87g H₂O.

So in this case, the two numbers are pretty close together, but we can see that the amount of water produced is limited by the amount of hydrogen - we'd need a little bit more to react with all of the oxygen remaining. All in all, our final amount would be 17.84 grams of water :)

Hope that helps!

-Niral