I know that answering these problems has to do with the use of ratios, but I honestly cannot understand how to solve them! Here is an example:


Consider this reaction:
4NH3 + 5O2 4NO + 6H2O
If you start with 1.5 moles of NH3 and 3 moles of O2, then:

How many moles of NO can be formed?

How many moles of H2O can be formed?


If you could explain step by step how to get these answers, that would be helpful!!

That's easy, you'll see.

a) Usually, you have:
4 moles of NH3 reacting with 5 moles of O2 giving 4 moles of NO and 6 moles of H2O

Ok?

So, concerning NH3 and O2, 4 moles react with 5 moles, correct?

Now, you start with 1.5 moles of NH3 and 3 moles of O2.

However, if you started with 1.5 moles of NH3, you need (1.5 * 5/4) moles of O2, which is 1.875 moles of O2.

Where did I get 5/4?
Well, 4 moles of NH3 react with 5 moles of O2.
1 mole of NH3 will react with 5/4 moles of O2
1.5 moles of NH3 will react with (1.5 * 5/4) moles of O2

Simple proportions! :)

So, you have an excess of O2, which will not react and 1.5 moles of NH3 which react.

4 moles of NH3 give 6 moles of H2O
1 mole of NH3 give 1 mole of NO
1.5 moles of NH3 give 1.5 moles of NO!

b) Since the number of moles of NH3 and O2 are the same as in a), you don't have to see which one is in excess.

4 moles of NH3 give 4 moles of NO
1 mole of NH3 give 6/4 moles of H2O
1.5 moles of NH3 give (1.5 * 6/4) moles of H2O

Is it clear now? :)

Still have problems, feel free to ask!