Drill – 2/7/11 What is the molar mass of BF 3 ? What is the molar mass of B 2 F 6 ? What is the ratio between these values?
Empirical Formulas
How would a scientist determine the identity of a substance that they made?
Percent Composition Mass of element in compound X 100% Total mass of compound Repeat for each element in compound Will add up to 100
Empirical Formula The symbols for the elements combined in the compound, with subscripts showing the smallest whole-number ratio of the different atoms in the compound. Ionic = formula unit Covalent = reduced molecular formula
If you know the percent composition of a compound, then you can determine its empirical formula. We do this by first assuming we have a g sample so that the percentage values can be converted to grams. We then convert the grams to moles (to account for atoms having different molar masses) To correct for our assumption of g we then divide each number of moles by the smallest number.
What is the empirical formula if a compound contains 32.4% Na, 22.6% S, & 45.0% O? Mass composition (in 100.0g) = 32.4 g Na, 22.6 g S, 45.0 g O Composition in moles = 32.4 g Na x (1 mol Na / g Na) = 1.41 mol Na 22.6 g S x (1 mol S / g S) = mol S 45.0 g O x (1 mol O / g O) = 2.81 mol O Smallest whole-number mole ratio 1.41 mol Na /.705 = 2.00 mol Na .705 mol S /.705 = 1.00 mol S 2.81 mol O /.705 = 3.99 mol O
Percent Composition Mass Composition in moles Smallest whole-number mole ratio
A compound’s empirical formula can also be used to determine its molecular formula. x (empirical formula) = molecular formula x = whole-number factor
Determine the molecular formula of the compound with an empirical formula of CH and a mass of amu. Molecular formula mass = amu Empirical formula mass = C + H = amu amu = amu x (empirical formula) = molecular formula x (13.02 amu) = amu x = amu / amu = So… CH C 6 H 6
In the green workbook: Pg 67 #1 b Pg 77 #1 a & b Pg 79 #1 a & b Pg 89 #1 a & b Pg 94 #1 a & b
Drill – 2/8/11 What is the percent composition of each element in Fe 2 O 3 ?