Drill – 2/7/11

What is the molar mass of BF3?

What is the molar mass of B2F6? 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 100.0 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 100.0 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 / 22.99 g Na) = 1.41 mol Na 22.6 g S x (1 mol S / 32.07 g S) = 0.705 mol S 45.0 g O x (1 mol O / 16.00 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

MassComposition

Composition in moles

Smallestwhole-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 78.11 amu.

Molecular formula mass = 78.11 amu Empirical formula mass =

C + H = 12.01 amu + 1.01 amu = 13.02 amu

x (empirical formula) = molecular formula

x (13.02 amu) = 78.11 amu

x = 78.11 amu / 13.02 amu = 5.999

So… CH C6H6

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 Fe2O3?