1. The Composition of Unknown Compounds Section 6.6

2. The Law of Definite Proportions The elements in a compound are always present in the same proportion by mass. Example: water is always H 2 O the proportion is always 2H:1O A combustion analysis can be used to determine the composition of an unknown organic compound. The percentage of carbon and hydrogen in the compound can be determined by analysing the amount of carbon dioxide and water generated by the combustion apparatus.

3. Percentage Composition is the percentage, by mass, of each element in a compound can be calculated from lab data or from the chemical formula % element = m element x 100% m sample

4. Example 1: From Lab Data A lab technician finds that an 11.5g sample of a liquid compound contains 6.00g of carbon and 1.51g of hydrogen. The remaining mass is oxygen. Determine the percentage composition of this compound.

5. G: m sample = 11.5 g m C = 6.00 g m H = 1.51 g m O = 11.5 g – 6.00 g – 1.51 g = 3.99 g R: % C = ? % H = ? % O = ? A: % element = (m element ÷ m sample ) × 100% S: % C = (6.00 g ÷ 11.5 g) × 100% = 52.2%

6. S: % H = (1.51 g ÷ 11.5 g) × 100% =13.1% % O = (3.99 g ÷ 11.5 g) × 100% = 35% P: The percentage composition of this compound is 52.2 % Carbon, 13.1% Hydrogen, and 35% Oxygen.

7. Example 2: From a Chemical Formula You can also calculate percentage composition from a chemical formula. For the mass of the element and the mass of the compound use the mass in u Determine the percentage composition of Ammonium nitrate

8. G: M NH4NO3 = 80.06 u M N = 28.02 u M H = 4.04 u M O = 48.00 u R: % N = ? % H = ? % O = ? A: % element = (M element ÷ M sample ) × 100% S: % N = (28.02 u ÷ 80.06 u) × 100% = 35.00% % H = (4.04 u ÷ 80.06 u) × 100% = 5.05%

9. S: % O = (48.00 u ÷ 80.06 u) × 100% = 59.96% P: The percentage composition of this compound is 35.00 % Nitrogen, 5.05% Hydrogen, and 59.96% Oxygen.

10. Application Alloys are mixtures of different metals blended together. The percentage composition of an alloy can be altered based on how it is going to be used. Example: Stainless steel is 10% chromium for cutlery and 18% chromium for surgical stainless steel. This difference makes the steel more resistant to corrosion. Fertilizers use different percentages of nitrogen, phosphorus and potassium to feed plants appropriately during different seasons

11. Homework p. 288 # 1 - 8