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Formula Weights

A simple but very important skill that you will be called upon to do many, many times during this course is to calculate the formula weight of a chemical based on its formula. You can do that very easily as long as you know the formula of the chemical with which you are dealing. I will present three examples (which are also shown in Example 12 in your workbook) and then have you try your hand at a few. If you are already familiar with determining formula weights, move on to the Quick Quiz on Calculating Formula Weights.

 

The first example is H2O. The formula weight is the sum of the atomic weights of everything that's in that particular formula. In the case of H2O, the formula weight of H2O is the sum of the weights of two hydrogens and one oxygen. To get the formula weight for H2O, you need twice the atomic weight of hydrogen and add to that the atomic weight of one oxygen. It is shown in the example that the formula weight of H2O is 18.0.
H2O 2 H 2 x  1.0 = 2.0
1 O 1 x 16.0 = 16.0
FW = 18.0

 

The next formula is HNO3. The formula contains 1 hydrogen, 1 nitrogen and 3 oxygens. So the weights come out to be 1.0 plus 14.0 plus 48.0, a total of 63.0.
HNO3 1 H 1 x  1.0 = 1.0
1 N 1 x 14.0 = 14.0
3 O 3 x 16.0 = 48.0
FW = 63.0


When you are dealing with parentheses, as in Fe(NO3)2, that's one more factor to take into account. However, you can easily interpret the formula to see that it represents 1 iron, 2 nitrogens and 6 oxygens. Then multiply the 1, 2, and 6 by the atomic weights for iron, nitrogen, and oxygen, add all those together and you get 179.8.
Fe(NO3)2 1 Fe 1 x 55.8 = 55.8
2 N 2 x 14.0 = 28.0
6 O 6 x 16.0 = 48.0
179.8
There is another way of doing this. That is to focus on what's in the parentheses. The NO3 portion of that compound has its own formula weight and the formula weight of NO3 comes out to be 62. (14.0 for the nitrogen and 3 times 16.0 gives 48.0 for the oxygen which comes to a total of 62.0.) This compound as a whole has 1 iron and 2 NO3's and so if you take the weight of the iron (55.8) and double the weight of the NO3 (which is 124.0) and add those together you come up with the formula weight of 179.8 which of course is the same as what we got using the first method.

 
(NO3) 1 N 1 x 14.0 = 14.0
3 O 3 x 16.0 = 48.0
FW of NO3 = 63.0
Fe(NO3)2 1 Fe 1 x 55.8 = 55.8
2 NO3 2 x 62.0 = 124.0
FW = 179.8

Something I should point out before we continue is that formula weights apply to any kind of formula. Whether you're dealing with molecular formulas or empirical formulas or whatever kind of formula you're dealing with you can calculate a formula weight for it. If you happen to be dealing with a molecular formula, the formula weight for it can also be called the molecular weight.

Practice Determining Formula Weights

Determining formula weights is very simple, straightforward, and important and I would like you to practice by determining the formula weights for the compounds in example 13 in your workbook.

So please take a moment to do that and check your answers below.  (If, after completing these, you would like to try some additional problems of this type, you will find several in a "Practice Problems" page in the Wrap-Up for this lesson.   Click here to go there now.)

Answers

For your answers you should have HF is 20.0; CH4 is 16.0; C3H7O2 is 75.0; Fe2(CO3)3 is 291.6. If you did not get these answers, check with the instructor to figure out why before continuing.

 

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E-mail instructor: Eden Francis

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