November 28th - Multistep Mole Conversions

Today, we took everything we know about moles and put it all together. We learned about multistep mole conversions, which are mole conversions that include more than one instance of dimensional analysis. Here are the conversions that we have learned so far:

That's pretty much it! Because we already know how to convert between mass, volume, molecules, atoms, and moles, we can dive right into the examples.

Ex. 1) If you have 13g of Cl₂ in a containter at STP, what is the volume of gas that you have?

Knowing that we have 13g of chlorine, and also knowing that moles are the central conversion factor, it is obvious that we must convert mass to moles to volume. The mass of 2 chlorine atoms in the diatomic molecule is (2)35.5 g/mol, or 71 g/mol.

13g   x   1 mol   x   22.4 L   = 4.1 L of chlorine gas

         71 g        1 mol                              

Ex. 2) A container holds 10.0 L of oxygen gas at STP. How many grams of oxygen is this?

Again, using our central conversion factor (the mole), we can convert this unit using molar volume and mass. The molar mass is (2)16.0 g/mol because oxygen is diatomic and the molar volume is 22.4 L/mol in all cases at STP. Let's solve:

10.0 L   x   1 mol   x   (2)16.0g   =   14.3 g of oxygen gas

22.4 L         1 mol                        

Ex. 3) A container holds 50.5 L of carbon dioxide. How many molecules of CO₂ is this? 

Now we have several things to take into consideration. First, we must account for the number of atoms and the different atoms in carbon dioxide. We must also realize that the question is asking for the number of molecules, not the number of atoms or the mass. Observe:

 50.5 L   x   1 mol   x   6.02 x 10²³ molec   =   1.36 x 10²⁴ molecules of carbon dioxide

    22.4 L             1 mol                                                               

It's just that easy! 

Now, with all of this, you can solve for any unit related to moles using dimensional analysis.

Here's a quick video for some more review:

Posted by Michael.