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?