Formula Weights (Molecular Mass) and Percent Composition

The molecular mass is the same numeric value as the atomic mass but the unit is different

Atomic mass is measured in atomic mass units (amu) and molecular mass is measured in grams

So molecular mass is the atomic mass with the unit “grams” or “g”

Molecular Mass

Chlorine has an atomic mass of 35.453 amu Therefore, its molar mass is… 35.453 grams The number is the same, just change the

unit

Molecular Mass

Formula weight of a compound is the sum of the atomic masses of the atoms as given in the formula of the compound. ◦ Sum of molecular masses

Formula Weight

Steps:1. Determine how many moles of each element

are in the compound using the subscripts.2. Find the molecular mass of each element in

the compound using #1 and atomic mass.3. Add it all together

We will use a table that looks like this…

Formula Weight

Element # Moles Atomic Mass

Molecular Mass

1. Determine moles first…◦ Subscripts

No parentheses- moles = subscripts parenthesizes– moles = outer subscript x inner subscript

Formula Weight~ Example

Element # Moles Atomic Mass

Molecular Mass

Aluminum 1

Bromine 3

2. To find the molecular mass of aluminum and bromine:

3. Add elements molar masses together to get compounds molar mass:

= 266.68 g/mol AlBr3

Formula Weight ~ Example

Element # Moles Atomic Mass Molecular Mass

Aluminum 1 26.98 g 26.98 g

Bromine 3 79.9 g 239.70 g

Element # Moles Atomic Mass Molecular Mass

Aluminum 1 26.98 g 26.98 g

Bromine 3 79.9 g 239.70 g

Percent composition is the percent that you find of any ONE element in a compound of elements.

Percent is found by taking the PART that you are looking for and dividing it by the WHOLE. ◦ Then multiply by 100 to move decimal.

Percent Composition

What percent of 92 is 42?◦In other words, the part “42” of the

whole “92” is what percent?◦42/92 x 100 = 45.7%

Percent Composition

What is the percent composition of calcium in calcium phosphate?

= 310.1828g /

mol

Percent Composition ~ Example

Element # Moles Molecular Mass

Ca 3 120.24 g

P 2 61.9476 g

O 8 127.9952 g

Now, take the PART (Ca) and divide it by the WHOLE. Multiple by 100.

= 310.1828g / mol

120.24 g Ca / 310.1828 g Ca3(PO4)2 x 100 = 38.8% Ca

Percent Composition ~ Example

Element # Moles Molecular Mass

Ca 3 120.24 g

P 2 61.9476 g

O 8 127.9952 g