Question

2H2 + O2 → 2H2O

How many moles of oxygen gas are required to produce 50.0 grams of water? (Keep 3 sig figs in your answer.)

Answer 1

Answer:

3 moles are required ....

Answer 2

Answer:

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Explanation:

Start by taking a look at the balanced chemical equation for this reaction

2H2(g]+O2(g]→2H2O(l]

Notice that you have a 2:1 mole ratio between hydrogen gas and oxygen gas.

This means that, regardless of how many moles of oxygen gas you have, the reaction needs twice as many moles of hydrogen gas in order to proceed.

You know that you start with 10.0 g of hydrogen gas nad 15.0 g of oxygen. To determine how many moles of each you have, use their respective molar masses

10.0g⋅1 mole H22.0159g=4.961 moles H2

and

15.0g⋅1 mole O232.0g=0.4688 moles O2

Notice that you have quite a significant difference, way bigger than the required 2:1 ratio needed, between how many moles of hydrogen gas and how many moles of oxygen you have.

This means that you're dealing with a limiting reagent. How many moles of oxygen would have been needed to react with all the hydrogen?

4.961moles H2⋅1 mole O22moles H2=2.4805 moles O2

Since you don't have that many moles of oxygen, it follows that oxygen is your limiting reagent, i.e. it will determine how much hydrogen reacts and how much remains in excess.

For example, how many moles of hydrogen gas would react if you have 0.4688 moles of oxygen?

0.4688moles O2⋅2 moles H21mole O2=0.9376 moles H2

The rest of the hydrogen will be in excess.