Mole-Mole Problems


Return to Stoichiometry Menu

The solution procedure used below involves making two ratios and setting them equal to each other. This is called a proportion. One ratio will come from the coefficients of the balanced equation and the other will be constructed from the problem. The ratio set up from data in the problem will almost always be the one with an unknown in it.

You will then cross-multiply and divide to get the answer.

What happens if the equation isn't balanced? Answer - balance it. You cannot do these problems correctly without a balanced equation. The ChemTeam is constantly amazed at the number of people who forget to balance the equation first.

How will I know which substances to use in the ratio? Answer - you will have to read the problem and understand the words in it.


Here is the first equation we'll use:

N2 + 3 H2 ---> 2 NH3

Problem #1: if we have 2.00 mol of N2 reacting with sufficient H2, how many moles of NH3 will be produced?

Solution Comments
  1. The ratio from the problem will have N2 and NH3 in it.
  2. How do you know which number goes on top or bottom in the ratios? Answer: it does not matter, except that you observe the next point ALL THE TIME.
  3. When making the two ratios, be 100% certain that numbers are in the same relative positions. For example, if the value associated with NH3 is in the numerator, then MAKE SURE it is in both numerators.
  4. Use the coefficients of the two substances to make the ratio from the equation.
  5. Why isn't H2 involved in the problem? Answer: The word "sufficient" removes it from consideration.

Let's use this ratio to set up the proportion:

That means the ratio from the equation is:

The ratio from the data in the problem will be:

The proportion (setting the two ratios equal) is:

Solving by cross-multiplying gives x = 4.00 mol of NH3 produced.


Problem #2: Suppose 6.00 mol of H2 reacted with sufficient nitrogen. How many moles of ammonia would be produced?

Let's use this ratio to set up the proportion:

That means the ratio from the equation is:

The ratio from the data in the problem will be:

The proportion (setting the two ratios equal) is:

Solving by cross-multiplying and dividing gives x = 4.00 mol of NH3 produced.


Problem #3: We want to produce 2.75 mol of NH3. How many moles of nitrogen would be required?

Before the solution, a brief comment: notice that hydrogen IS NOT mentioned in this problem. If any substance ISN'T mentioned in the problem, then assume there is a sufficient quantity of it on hand. Since that substance isn't part of the problem, then it's not part of the solution.

Let's use this ratio to set up the proportion:

That means the ratio from the equation is:

The ratio from the data in the problem will be:

The proportion (setting the two ratios equal) is:

Solving by cross-multiplying and dividing (plus rounding off to three significant figures) gives x = 1.38 mol of N2 needed.


Practice Problems

Here's the equation to use for all three problems:

2 H2 + O2 ---> 2 H2O

1) How many moles of H2O are produced when 5.00 moles of oxygen are used?

2) If 3.00 moles of H2O are produced, how many moles of oxygen must be consumed?

3) How many moles of hydrogen gas must be used, given the data in problem two?

This question just slipped in here somehow. If you can't answer it right now, that's OK. Go to the solution and read it carefully.

4) Suppose 4.00 grams of H2 were used? How many grams of water would be produced? Here are two links intended to give you a hint as to how to solve this problem:

Convert Grams to Moles.
Convert Moles to Grams.

Answers

Return to Stoichiometry Menu