# of atoms when combining elements

[Feodalherren]

Homework Statement

One liter of nitrogen combines with 3 L of hydrogen
to form 2 L of ammonia. If the molecules of nitrogen
and hydrogen have two atoms each, how many atoms of
hydrogen and nitrogen are in one molecule of ammonia?

Homework Equations

Chemical equations

The Attempt at a Solution

I didn't know where to start so I looked at the solution key and got this:

The number of liters tells us how many molecules of the original combine to form this number of molecules of the product. Using these numbers as multipliers in a chemical equation gives us

1N₂ + 3H₂ → 1N_x H_y

Making sure that we have the same number of atoms of each kind on each side, we get x = 1 and y = 3.

Can somebody explain to me what the hell just happened?

 

[frogjg2003]

Try asking in the "Other Science" homework forum. While most of us here are capable of helping, this is a chemistry, not physics question.

As for what happened:
What is the relation between number of molecules and volume, given constant pressure and temperature?

 

[Nugatory]

You have two liters of ammonia, not one. So something is wrong with the right-hand side of the reaction as you've written it. Fix that and it will make sense.

 

[Feodalherren]

Sorry, this question came from my physics HW so I posted it here.
Maybe a moderator can move it.

Even when I change it to 2L it makes no sense to me. I haven't taken chemistry since high school.

 

[frogjg2003]

There are the same number of nitrogen molecules in 1L of nitrogen as there are hydrogen molecules in 1L of hydrogen and ammonia molecules in 1L of ammonia.
A nitrogen molecule is composed of two nitrogen atoms and a hydrogen molecule is composed of two hydrogen atoms. An ammonia molecule is composed of (for you) an unknown number of nitrogen and hydrogen atoms. That's where x and y came from.
You know that 1 part nitrogen molecule and 3 parts hydrogen molecule becomes 2 parts ammonia molecule. So [itex]1\times 2[/itex] parts nitrogen atoms becomes [itex]2\times x[/itex] nitrogen atoms and [itex]3\times 2[/itex] parts hydrogen atoms becomes [itex]2\times y[/itex] hydrogen atoms.

 

[Feodalherren]

Aaaah I see! Much appreciated Sir!