November 16th - Mole Conversions

Today we learned all about mole conversions! Mole conversions are the process in which dimensional analysis is used to convert between moles and grams. 

To convert between moles and mass in grams, we use molar mass as the conversion factor. Molar mass can be found on the periodic table (it is the same as atomic mass). In a compound, it is the sum of the masses of all atoms. For example, if you were only using oxygen, the molar mass would be 16.0 g/mol (the molar mass of oxygen). If you were using carbon dioxide, the molar mass would be 12.0 g/mol + (2)16.0 g/mol for a total of 44.0 g/mol. 

Remember to cancel out the appropriate units and remember significant digits!

Here are a few examples:

Ex. 1) How many grams are there in 1.2 mol of chlorine gas?

First, start out by writing what you know. We know that there's 1.2 mol and that there are 2 oxygen atoms in one compound. We can also look up the atom weight, which is 16.0 g/mol. Then, remembering that the unit we are converting to always goes on top, we can calculate:

1.2 mol   x   (2)35.5g   =   85g of chlorine gas

1 mol                  

Ex. 2) How many moles are there in 100g of Fe₃O₄? 

Knowing that we have 100g, that the molar mass of iron is 55.5 g/mol and that the molar mass of oxygen is 16.0 g/mol, we can find the number of moles:

100g                      x                      1 mol                     =                     0.433 mol of Fe₃O₄

(3)55.5 g + (4)16.0 g                   

Here's a video providing another example of such conversions:

Ex. 3) How many grams are needed to make 3.0×10^-3mol ofMgCO₃ ·5 HOH?

We know that we have 3.0 × 10^-3 mol of the substance, that the molar mass of magnesium is 24.3 g/mol, of carbon is 12.0 g/mol, of oxygen is 16.0 g/mol, and of hydrogen is 1.0 g/mol. We add the molar mass of the hydrate to that of the compound in this equation. Using this knowledge, we can solve:

 3.0 × 10^-3 mol   x   24.3 g/mol + 12.0 g + (8)16.0 g + (10)1.0 g   =   0.52g of MgCO₃ ·5 HOH

 1 mol               

 Ex. 4) A compound, made of phosphorus and chlorine, contains 0.200 mol and has a mass of 27.5g. Determine the molar mass and suggest a possible formula.

We know that molar mass is stated in the units of g/mol. We know both the number of mols and the mass, so we can find the molar mass:

27.5g /0.200 mol = 137.5 g/mol

To find the formula that corresponds to this molar mass, we can create a chart:

With this, we know that the compound contains 1 phosphorus atom and 3 chlorine atoms. Therefore, the compound is PCl₃.

That was fun, wasn't it? Here's another video (if you need more practice!):

Posted by Michael.