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Formula Weights (Molecular Mass) and Percent Composition. Molecular Mass. 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 - PowerPoint PPT Presentation
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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
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