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Formula Weights (Molecular Mass) and Percent Composition

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

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Formula Weights (Molecular Mass) and Percent Composition

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  1. Formula Weights (Molecular Mass) and Percent Composition

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

  3. 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

  4. Formula Weight • 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

  5. Formula Weight • Steps: • Determine how many moles of each element are in the compound using the subscripts. • Find the molecular mass of each element in the compound using #1 and atomic mass. • Add it all together • We will use a table that looks like this…

  6. Formula Weight~ Example • 1. Determine moles first… • Subscripts • No parentheses- moles = subscripts • parenthesizes– moles = outer subscript x inner subscript

  7. Formula Weight ~ Example • 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

  8. Percent Composition • 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.

  9. 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%

  10. Percent Composition ~ Example • What is the percent composition of calcium in calcium phosphate? = 310.1828g / mol

  11. Percent Composition ~ Example • 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

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