# Chemistry moles question

Hi please could i have help on this question? I don’t understand how statement 3 is correct? I wrote the balanced chemical question and found that 2 moles of H2S reacts with 3 moles of oxygen. I watched a video that then said that H2S is in excess but surely it is limiting because there are less moles?
Here is the question: https://app.gemoo.com/share/image-annotation/634614966315048960?codeId=DGWrXmJRbGNdb&origin=imageurlgenerator

Thank you!!
(edited 1 month ago)
Original post by anonymous294
Hi please could i have help on this question? I don’t understand how statement 3 is correct? I wrote the balanced chemical question and found that 2 moles of H2S reacts with 3 moles of oxygen. I watched a video that then said that H2S is in excess but surely it is limiting because there are less moles?
Here is the question: https://app.gemoo.com/share/image-annotation/634614966315048960?codeId=DGWrXmJRbGNdb&origin=imageurlgenerator
Thank you!!

The equation for the reaction is:

2H2S + 3O2 ==> 2SO2 + 2H2O

If there is equal moles of hydrogen sulfide and oxygen then not all of the hydrogen sulfide can react - it is in excess.

Ask yourself the question: "if all of the hydrogen sulfide reacts how many moles of oxygen does it need?"

Imagine that you have 2 mol of both reactants. The hydrogen sulfide would need 3 mol of oxygen, but there is not enough, therefore not all the hydrogen sulfide can react. 2 mol of H2S needs 3 mol of oxygen for complete reaction.

Now ask the second question. If I have 2 mol of oxygen how many mol of hydrogen sulfide can it react with?
The answer here is less then 2 mol, so there must be some hydrogen sulfide left over.
Original post by charco
The equation for the reaction is:
2H2S + 3O2 ==> 2SO2 + 2H2O
If there is equal moles of hydrogen sulfide and oxygen then not all of the hydrogen sulfide can react - it is in excess.
Ask yourself the question: "if all of the hydrogen sulfide reacts how many moles of oxygen does it need?"
Imagine that you have 2 mol of both reactants. The hydrogen sulfide would need 3 mol of oxygen, but there is not enough, therefore not all the hydrogen sulfide can react. 2 mol of H2S needs 3 mol of oxygen for complete reaction.
Now ask the second question. If I have 2 mol of oxygen how many mol of hydrogen sulfide can it react with?
The answer here is less then 2 mol, so there must be some hydrogen sulfide left over.

Ohh this makes much more sense, thank you