A. Calculation of Formula Weights
The formula weight (sometimes called the molecular weight) of a compound or ion is the sum of the atomic weights of all elements in the compound or ion, with each element's atomic weight being multiplied by the number of atoms of that element occurring in the formula.

B. Moles of Compounds
In Section 4.4 the mole was defined as 6.02 X 10²³ items. In that section, we talked about moles of atoms and you learned that one mole of a particular element has a mass in grams equal to the element's atomic weight. We can also have moles of compounds or of ions. One mole of a compound or an ion will have a mass in grams equal to the formula weight of that compound or ion. In Example 6.8 we calculated the formula weight of sulfuric acid to be 98.09 amu, thus it follows that one mole of sulfuric acid has a mass of 98.09 g. The formula of a compound also tells how many moles of a particular element are contained in one mole of the compound. For example, one mole of sulfuric acid contains:

2 mol hydrogen atoms, weighing 2(1.008 g) or 2.016 g

1 mol sulfur atoms, weighing 1(32.07 g) or 32.07 g

4 mol oxygen atoms, weighing 4(16.00 g) or 64.00 g

Note that these individual masses add up to our calculated formula weight for the acid. This relationship between mass and moles of a compound is often used as a conversion factor in solving problems.

 

Example: How many atoms of oxygen are in 0.262 g carbon dioxide, CO2? Wanted: ? atoms oxygen Given: 0.262 g carbon dioxide Conversion factors The formula weight of carbon dioxide is: carbon 1x12.01 =  12.01 Oxygen 2x16.00 = 32.00 Total:                     44.01 amu (for CO2) Equation Answer: 7.17x1023 atoms O

In this example , notice that each factor involving a mole states the chemical composition of the mole: "1 mol CO₂" and "2 mol O atoms." As problems become increasingly complex, this bookkeeping habit becomes especially important. Notice also that the example deals with atoms of oxygen; we are not concerned here with the fact that oxygen exists in nature as a diatomic molecule, O₂.

C. Percent Composition
Percent means parts per hundred. The percent composition of a compound is the number of grams of each element or group of elements in 100 g of the compound, expressed as a percent.

Percent element

=

g element g compound

X

100%

For example, the percent composition of potassium chloride, KCl, can be calculated from the atomic weights of potassium and chlorine and the formula weight of KCl.

Formula weight of KCl: 39.10 g + 35.45 g = 74.55 g

Percent potassium:

39.10 g K 74.55 g KCl

X 100% = 52.45% potassium

Percent chlorine:

35.45 g Cl 74.55 g KCl

X 100% = 47.55% chlorine

Example: Calculate the percent composition of carbon tetrachloride, CCl4 Solution The formula weight of carbon tetrachloride is: carbon 12.01 x 1 = 12.01 chlorine 35.45 x 4 = 141.8     153.8 amu Percent C 12.01 g C 153.8 g CCl4 x100% = 7.81%C Percent Cl 141.8 g Cl 153.8 g CCl4 x100% = 92.19%Cl It is always wise to chck these percent calculations by assuring yourself that they add up to 100% as they do here.