Molar Mass

Molar Mass, as you've probably guessed, is the mass of 1 mole of a substance!
 It can be determined from the atomic mass on the periodic table

It's measured in grams per mole or g/mol.
I like the way that it is described in this phrase:
The atomic weight of an element expressed in grams contains 1 mole of that element.

To determine the molar mass of a compound, find the mass of each element individually and add them!
For example

NO2.  N= 14 amu, so 14 g
           O= 16 amu, so 16 g

           14+2(16)= 46 g/mol

What is the molar mass of Ammonium Phosphate?
Ammonium Phosphate is (NH4)3PO4
N= 14g x 3 = 42 g

H= 1g x 12 = 12 g

P= 31 g

O= 16 g x 4 = 64 g
42 + 12 + 31 + 64 = 149 g

Converting between Moles and Mass

To convert between moles and mass we use molar mass as the conversion factor. Make sure to cancel the appropriate units!

Examples:
How many grams are there in 1.5 mol of O2?

We are trying to cancel out mol, and convert it into grams, so when converting, we will put mol on the bottom, and grams on the top.

1.5 mol O2 x                   g   =               g

                           1 mol

We have to find the molar mass of O2, in grams. O= 16     16 x 2 = 32.0 g

1.5 mol O2 x 32.0 g    = 48 g

                      1 mol

How many moles are present in Fe2O3?

115 g Fe2O3 x 1 mol  =           mol
                               g

Fe= 55.8 x 2 = 111.6

O= 16 x 3 = 48 g                 111.6 g + 48 g = 159.6 g   * at this point, don't worry about sig fig's!

115 g Fe2O3 x 1 mol       =   0.721 mol Fe2O3

                         159.6 g

Here's another kind, that has more super cool detective problem solving:

A compound is made up of phosphorous and chlorine. It is found to contain 0.200 mol and has a mass of 27.5 g. Determine the molar mass, and a possible formula for this compound.

molar mass = g      = 27.5  g      =  137.5 g/mol          

                     mol      0.200 mol

To get the formula, you just have to try out different masses off the periodic table, and the one that works in this case is PCl3!

I wonder what the molar mass of this is?

Chemistry Pun of the day:

Q: What weapon can you make from the Chemicals Potassium, Nickel and Iron?
A: KNiFe.