1. PERCENT COMPOSITION AND CHEMICAL FORMULAS Section 10.3 Part #1

2. Percent Composition - Definition  Percent Composition – the percent by mass of each element in a compound  Expresses the relative amounts of each element in a compound  There are as many % values as there are different elements in the compound

3. Percent Composition - Definition  % Composition of an Element (E) = (grams of E ÷ grams of compound) x 100%  The Percent Composition of all elements in the compound added up together must = 100%

4. Ex. K 2 CrO 4 Ionic Compound, Molecular Compound, or Acid?  Ionic Name?  Potassium Chromate Made up of:  K = 40.3%  Cr = 26.8%  O = 32.9% 40.3% + 26.8% + 32.9% = 100.0% But how do YOU find these percentages?

5. Percent Composition from Mass An 8.20 g piece of Magnesium combines completely with 5.40 g of Oxygen to form a compound. What is the % Composition of this compound?  Known:  Mass of Mg = 8.20 gMass of O = 5.40 g  Total mass of compound = 8.20 g + 5.40 g = 13.60 g  % Mg = mass of Mg/mass of compound x 100% = 8.20g Mg / 13.60 g Compound x 100% = 60.3 % Mg  % O = mass of O / mass of compound x 100% = 5.40 g O / 13.60 g Compound x 100% = 39.7% O  Check: 60.3 % Mg + 39.7 % O = 100.0%

6. Percent Composition from Formula  To calculate the % Composition of a known compound, use the chemical formula to calculate the molar mass  This gives the mass of 1 mole of the compound  Next calculate the % by mass of 1 mole of the compound for each element  Divide the mass of each element in one mole of the compound by the molar mass and multiply by 100%  % mass = grams of element in 1 mol compound ÷ molar mass of compound x 100%

7. Example Calculate the % Composition of Propane (C 3 H 8 )  Find the molar mass of C 3 H 8  3(12.01gC) + 8 (1.01gH)  36.03 g C + 8.08 g H = 44.11 g C 3 H 8  Mass of C in 1 mol C 3 H 8 = 36.03 g C  Mass of H in 1 mol C 3 H 8 = 8.08 g H  % C = Mass of C / g of Compound x 100%  = 36.03 g C / 44.11 g C 3 H 8 x 100% = 81.68% C  % H = Mass of H / g of Compound x 100%  = 8.08 g H / 44.11 g C 3 H 8 x 100% = 18.32% H

8. Using % as a Conversion Factor  You can use % Composition to calculate the # of grams of an element in a specific amount of a compound.  Multiply the mass of the compound by the conversion factor based on % Composition.

9. From the Previous Example  Ex. Propane: C 3 H 8  81.68% C, 18.32%H  How many grams of Carbon would you have in a 100g sample of C 3 H 8 ?  81.68% C is in C 3 H 8 is like saying 816.8g C/100g C 3 H 8  100g C 3 H 8 (81.68 g C/100 g C 3 H 8 ) = 81.68 g C  You can use this relationship as a conversion factor. We will use it in the next example.

10. Example Calculate the mass of Carbon in 82.0 g of Propane (C 3 H 8 )  Known:  Mass of C 3 H 8 = 82.0 g  % Mass of C in C 3 H 8 = 81.68%  We need to find mass of C in g  82.0 g C 3 H 8 (81.68 g C / 100g C 3 H 8 ) = 67.00 g C  Check:  67.00 g C/82.0 g C 3 H 8 = 81.68% C