percent composition & empirical formula: chemistry
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Percent Composition & Chemical Formulas
Percent Composition of a Compound
• Definition: The percent by mass of an element in a compound
• Ways to calculate % composition by mass:– 1) mass of element x 100% mass of compound
– 2) mass of element in 1 mol of compound x 100% molar mass of compound
Example Problem #1:
• Calculate the percent composition by mass of nitrogen in the compound NH3?– 1) mass of element x 100% mass of compound
• 14g x 100 % = 82.4% N in NH3
17g
Example Problem #2:• Calculate the percent by mass of carbon in
propane, C3H8?– 2) mass of element in 1 mol of compound x 100% molar mass of compound
• Mass of C in 1 mol of C3H8 = 36g x 100= 82% C
Molar mass of C3H8 44g
Empirical Formula• Gives the lowest whole-number ratio of the
atoms or moles of the elements in a compound
Molecular Formula• Either the same as the empirical formula OR a
simple whole-number multiple of the empirical formula
• Chemists use mass spectrometer to determine molar mass
Example Problems:• Reduce the molecular formulas to the
empirical formulas by dividing by the smallest whole number ratio
• 3. P4O10
• 4. N2O4
• 5. C6H8O3
• 6. C100H220
Calculating Empirical Formula• The percent composition of a compound can be
used to calculate the empirical formula of the compound
• Steps:• 1) convert mass in grams to moles
• 2)Divide each molar amount by the smaller # of moles
• 3) Multiple each part of ratio by smallest whole # so subscripts are whole #s
Calculating Empirical Formula:• 7. Calculate the empirical formula of a compound
that is 25.9% Nitrogen and 74.1% Oxygen.
Hints• On these stoichiometric problems, ask
yourself:–What are you solving for? –What quantity is given?
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