How many molecules of each reactant are remain after the reaction is complete? What is the limiting reactant? How many molecules of NH3 are produced?

Question

Nitrogen and hydrogen combine at a high temperature, in the presence of a catalyst, to produce ammonia.

N2 (g) + 3H2 (g) = 2NH (g)

There are four molecules of nitrogen and nine molecules of hydrogen present in the diagram.

When the reaction is complete, how many molecules of NH3 are produced?

What is the limiting reactant?

How many molecules of each reactant are remain after the reaction is complete?

Answer 1

You do not actually need the diagram - the text gives you the two counts, four N2 and nine H2. And the equation lost a subscript in the posting; it is:

N2 + 3 H2 -> 2 NH3

The nice thing about a molecule-counting problem is that you can reason in whole molecules and skip moles entirely. The coefficients are a recipe: one N2 plus three H2 makes two NH3.

Which reactant runs out first

Ask what each reactant could produce if it were used up completely.

4 N2 x (2 NH3 / 1 N2) = 8 NH3
9 H2 x (2 NH3 / 3 H2) = 6 NH3

The smaller number wins, so H2 is the limiting reactant and nitrogen is in excess. You can see the same thing directly: four N2 would demand 4(3) = 12 H2, and only nine are there.

How much ammonia

Use the limiting reactant only: 6 molecules of NH3.

What is left over

All nine H2 are consumed, so 0 H2 remain.

The nitrogen consumed is 9 H2 x (1 N2 / 3 H2) = 3 N2, so 4 - 3 = 1 N2 molecule remains.

Check it with atoms

Six NH3 contain 6 N and 18 H. The 3 N2 you used up supplied 6 N, and the 9 H2 supplied 18 H. Everything balances, and the one leftover N2 is untouched. Counting atoms on both sides is the fastest way to catch an error on these particle-diagram problems, and it works even when you cannot see the picture.