CHEMISTRY 10/31/11

Brainteaser 10/31/11 Make sure you’re on the right side! –Week 3 –Mon 10/31/11 Find the percent composition by mass of each element in this compound: –NaCl *HINT: mass of element/mass of compound *100 = percent composition)

Agenda Warm Up Lecture: –Percent Composition, Empirical and Molecular Formula Homework: –Worksheet

Unit 6: Percent Composition Percent Composition –- –Used to figure out how much of each mass of each element is contained in a compound –Example A 100-g sample of a new compound contains 55-g of element X and 45-g of element Y. What is the percent by mass of each element?

Percent Composition Percent composition from the chemical formula –For example: Determine the percent composition of the elements in water 1 st : Determine the chemical formula of water 2 nd : Calculate the molar mass of water (18 g/mol) assume you have a 18 g sample 3 rd : Determine how many of each element is in the compound and find it’s mass 4 th : Divide each element by the molar mass to get the percent by mass. Mass of element in 1 mol compound x 100 = % by mass molar mass of compound

Practice Problem Determine the percent by mass of each element in calcium chloride

Empirical Formula Relying on or derived from observation/experiment Formula with the smallest whole # mole ratio of the elements Determine the chemical formula by % composition If % comp. is given assume the mass is g and the % by mass of each element is equal to the mass of that element in grams

Empirical Formula For example: –The percent composition of an oxide of sulfur is 40.05% S and 59.95% O –Thus, 40.05g S and 59.95g O –Convert to the number of moles –The mole ratio of S atoms to O atoms in the oxide is ________ : _________ –Convert these ratios to whole numbers by dividing each ratio by the smallest ratio between the two elements: –Mole ratio of S to O is ____: ____ = ______

Empirical Formula Empirical formula-lowest ratio of elements Can be determined if we know the masses of each element in a sample of the compound OR Percent composition of each element in the compounds –If the masses are known, follow these easy conversions Mass  Moles  Mole ratio  Formula If the percent compositions are known, simply assume you have a 100 gram sample, and then follow the same steps as shown above. Remember to write the least electronegative element first Mole ratio tells you what the subscripts on each element should be.

Practice Problem A 9.2 gram sample of a compound has 2.8 grams Nitrogen and 6.4 grams Oxygen. Determine the empirical formula of the compound.