In today's class we pulled several different things together. We applied the concept of a mol to chemical equations to demonstrate what moles were used for, and what a balanced chemical equation means. We began with the formation reaction for water.

2H₂(g) + O₂(g) ------> 2H₂O(l)

We read this equation...2 molecules of H₂(g) react with one molecule of O₂(g) to form 2 molecules of H₂O(l). We can also read this equation 2 mol H₂(g) react with 1 mol O₂(g) to form 2 mol of H₂O(l). The balanced equations tells us the relationship between pairs of substances. For example according to the balanced equation 2 mol of H₂(g) react with one mol of O₂(g)_; also 2 mol of H₂(g) are required to form 2 mol of H₂O(l); and 1 mol of O₂(g) is required to form 2 mol of H₂O(l). We can write all three of these relations in a ratio form as shown below;

we can use these ratioes to convert from moles of one substance to moles of another substance. Let's consider the examples we did in class today;

Sample 1:

How many moles of H₂ are required to produce 4.0 moles of H₂O?

According to the balanced chemical equation 2 mol of H₂O are produced when 2 mol H₂ react. So we can express this using the ratio as shown above to convert moles of H₂O to moles of H₂.

Sample 2:

How many moles of O₂ are required to produce 4.0 moles of H₂O?

According to the balanced chemical equation 2 mol of H₂O are produced when 1 mol O₂ react. So we can express this using the ratio as shown above to convert moles of H₂O to moles of O₂.

Sample 3:

How many moles of H₂ are required to produce 0.835 moles of O₂?

According to the balanced chemical equation 2 mol of H₂ react with 1 mol O₂. So we can express this using the ratio as shown above to convert moles of H₂O to moles of O₂.

We next looked at a new formation equation;

3H₂(g) + N₂(g) ------> 2NH₃(g)

read this equation...3 mol of H₂(g) react with one mol of N₂(g) to form 2 molecules of NH₃(g). The balanced equations tells us the relationship between pairs of substances. For example according to the balanced equation 3 mol of H₂(g) react with one mol of N₂(g)_; also 3 mol of H₂(g) are required to form 2 mol of NH₃(g); and 1 mol of N₂(g) is required to form 2 mol of NH₃(g). We can write all three of these relations in a ratio form as shown below;

we can use these ratioes to convert from moles of one substance to moles of another substance. Let's consider the examples we did in class today;

Sample 4:

How many moles of H₂ are required to produce 27 moles of NH₃?

According to the balanced chemical equation 2 mol of NH₃ are produced when 3 mol H₂ react. So we can express this using the ratio as shown above to convert moles of NH₃ to moles of H₂.

Sample 5:

How many grams of N₂ are required to react with 2 moles of H₂?

According to the balanced chemical equation 3 mol of H₂ react with 1 mol N₂. So we can express this using the ratio as shown above to convert moles of H₂ to moles of N₂.

But the question is asking for grams of N₂ not mol! So we need to convert from grams N₂ of to mol of N₂. We do that using the molar mass of N₂.

Sample 6:

How many grams of NH₃ are formed when 5.0 g H₂ react with plenty of N₂?

According to the balanced chemical equation 3 mol of H₂ react to form 2 mol NH₃. So we can express this using the ratio as shown above to convert moles of H₂ to moles of NH₃. But in this case I gave you the amount of in H₂ in grams, not mol! But we have to have the amount of H₂ in mol to do this problem, so lets start by converting grams of H₂ to mol of H₂.

Now we can use the information in the balanced chemical equation to convert from mol H₂ of to mol of NH₃.

Finally we will convert the mol NH₃ of to grams using the molar mass of NH₃.

Here are some additional problems to try that are like these...