| This is probably the single most used equation in chemistry. It is the one that allows the conversion of moles into grams and grams back into moles. Moles are a "theoretical value" which looks good on paper. Grams is the "practical value" that you take out of a stock bottle and place on the balance. Lab balances read in grams, not moles, and so in practical work all mole values have to first be converted. |
| Moles to grams formula: grams(g) = moles(n) X molecular mass |
| Example: Sodium bicarbonate, NaHCO3, is one ingredient
of baking powder. How many grams of sodium bicarbonate are in 0.673
moles? |
| Step 1. Find the molecular mass: NaHCO3 = 1 Na = 1 X 22.99 g/mol = 22.99 g/mol 1 H = 1 X 1.01 g/mol = 1.01 g/mol 1 C = 1 X 12.01 g/mol = 12.01 g/mol 3 O = 3 X 16.00 g/mol = 48.00 g/mol 84.01 g/mol |
| Step 2. Use the equation: g = n X molecular
mass = 0.673 mol X 84.01 g mol = 56.54 g |
| Step 3: Provide a written answer: There are 56.54 grams of sodium
bicarbonate in 0.673 moles. |
| Grams to moles formula:
moles =
grams OR n
= g
|
| Example: Potassium permanganate, KMnO4, at one time
was used for an anti-fungal agent. You could always tell someone who had just been treated because their feet were purple. The pharmacy gives you 250 grams of the stuff in a bottle. How many moles of it do you have? |
| Step 1: Find the molecular mass of KMnO4: 1 K = 1 X 39.10 g/mol = 39.10 g/mol 1 Mn = 1 X 54.94 g/mol = 54.94 g/mol 4 O = 4 X 16.00 g/mol = 64.00 g/mol 158.04 g/mol |
| Step 2: Use the equation: n =
g
= 250 g
= 1.58 mol molecular mass 158.04 g/mol |
| Step 3: Write a written answer. A sample of 250 grams is equivalent to 1.58 moles of potassium permanganate. |
| Finding the molecular mass |
| In the above two examples you have had a molecular formula to work
with. As long as you have a molecular formula, or name, from which you
can make a formula up, you will always have a molecular mass.
In order to find the molecular mass, without a name to go on, you need two
pieces of information. The number of grams and the number of moles that it
represents. |
| molecular
mass = grams =
g moles n |
| Example: A fellow student comes to you with a sample of an unknown chemical. They tell you that it has a mass of 34.91 grams and that it is exactly 0.20 moles. What is the molecular mass of the substance? |
| Step 1: Use the equation: molecular
mass = 34.91 g = 174.55
g 0.20 mol mol |
| Step 2: Write a sentence. The unknown substance has a molecular mass of 174.55 g/mol. |