Stoichiometry with molarity and gases

Let’s look at two more stoichiometry problems, one involving molarity and one involving gases.

Remember that ALL stoichiometry problems have three steps, which are:

1. Go to moles

2. Use the reaction fraction

3. Go to the unit you need

Let’s try a problem with molarity.

If you don’t remember molarity, make sure you remind yourself here and here before you look at this problem.

How many grams of barium phosphate are produced when 13.9 mL of 1.07 M potassium phosphate react with barium nitrate according to the reaction below?

3 Ba(NO3)2 + 2 K3PO4 → 6 KNO3 + Ba3(PO4)2

First, as before, we’ll put the relevant information below the compounds in the reaction

Now we’ll start the problem with the given information on the left side. Remember, we NEVER start a problem with molarity because it is a ratio, not an amount.

The first step (as listed above)is to convert to moles. This will actually require two fractions for the one step.

Now we’re ready for the reaction fraction to change from K3PO4 to Ba3(PO4)2.

Finally, the last step (as above) asks us to convert to the unit we need, which in this case is grams. For that we use molar mass.

And, of course, now we can solve the problem.

Let's try a problem involving gases

In order to do this problem, you need to know the ideal gas law. If you've forgotten it or just need a refresher, look here.

How many liters of oxygen gas at 387 K and 2.55 atm are needed to burn 5.44 grams of propanol?

As always, we'll write out the balanced reaction and put the relevant information under the appropriate compounds.

This problem will look a little different (because it involves gases), but if you remember the three steps, you'll see that it still follows the same pattern as the other problems. Those steps, once again, are:

1. Go to moles

2. Use the reaction fraction

3. Go to the unit you need

The first two steps here will look like every other problem we've done:

Now, we need to pause and think. We can't just add a last fraction because that's not the way gases work. When we deal with gases and moles, we have to use the ideal gas law.  Since we want V, we'll  rearrange 

and we'll need to know T, P and n. We were given temperature and pressure in the problem. But we need moles to solve it.

Good news! After those first two steps we have the moles of O2. So, we can do the math for the first two steps

then finish with the ideal gas lab