molecular and empirical formulas percentage composition: mass of each element compared to the mass...
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Molecular and Empirical Formulas
Percentage Composition:• Mass of each element compared to the mass of the compound
(m/m) or volume of each compared to the total volume (v/v)
Method (m/m) • Find the mass (molar or given) of the sample • Divide the mass of each atom by the mass of the sample
weight • Multiply by 100% • The total of all percentages must equal 100%
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Molecular Formula• Formula that gives the exact number of elements in a molecule or
compound. • Find the % Composition for each element in the compound.
– If given % Composition: start with the % composition listed as decimals• Find the contributing mass of each element
– Multiply each % Composition by the sample weight of the compound to find the contributing mass
• Do a mass to mole conversion for each element • Assemble the formula
– The moles of each become the new subscripts of your compound
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Empirical Formula• Same steps as finding the molecular formula• Find the sample weight of the compound • Use % composition to find the contributing mass• Do the mass to mole conversion for each element • Divide all by the smallest and round to whole numbers • Assemble the formula
– The ratios expressed as whole numbers become the subscript numbers of the atoms