1. 3.10 Determining a Chemical Formula from Experimental Data

2. Analysis of a Compounds Constituent Elements
The mass of elements that make up a compound can be used to determine the compounds chemical formula.
When determining a compounds empirical formula from mass data:
1st convert mass to moles. (If % comp. is given, convert to mass.)
2nd divide all subscripts (number of moles) by the smallest one to determine the number of atoms of each element.
3rd we must make the number of atoms of each element a simple whole number.

3. Let’s Try a Sample Problem
A sample of a compound is decomposed in a laboratory and produces 165 g carbon, 27.8 g hydrogen, and g oxygen. Calculate the empirical formula of the compound. Step 1: 165 g C g C = X = 13.7 mol C X 1.00 mol 27.8 g H 1.01 g H = X = 27.5 mol H X 1.00 mol g O g O = X = 13.7 mol O X 1.00 mol O

4. Sample Problem 1 (Continued)
A sample of a compound is decomposed in a laboratory and produces 165 g carbon, 27.8 g hydrogen, and g oxygen. Calculate the empirical formula of the compound mol C = 1 mol C mol H = 2 mol H 13.7 mol O = 1 mol O This compounds empirical formula is: CH2O

5. Calculating Molecular Formulas for Compounds
If we know the empirical formula of a compound and its molar mass, we can calculate the molecular formula.
Since a molecular formula is a whole number multiple of its empirical formula, we can calculate the simple whole number multiple (n) by:
molar mass
n =
empirical formula molar mass
Next, all we have to do is multiple the empirical formula by (n) to determine the molecular formula of the compound.
Molecular formula = empirical formula X n

6. Let’s Try a Sample Problem
A compound with the percent composition shown next has a molar mass of g/mol. Determine its molecular formula. C = 39.97% , H= 13.41%, and N= 46.62% Step 1: Convert % to mass 39.97% C = g 13.41% H = g 46.62% N = g Step 2: Convert mass to moles (I will use dimensional analysis this time) 1.00 mol C g C X = 3.33 mol C g C 1.00 mol H g H X = 13.3 mol H 1.01 g H 1.00 mol N g N X = 3.33 mol N g N

7. Sample Problem 2 (Continued)
A compound with the percent composition shown next has a molar mass of g/mol. Determine its molecular formula. Step 3: Convert mol to atoms 3.33 mol C = mol 13.3 mol H = mol N The empirical formula of this compound is: CH4N (Now we must determine its molecular formula)

8. Sample Problem 2 (Continued)
A compound with the percent composition shown next has a molar mass of g/mol. Determine its molecular formula. Step 4: Determine molecular formula molar mass g/mol n = = = 2 empirical formula molar mass g/mol Molecular formula = empirical formula X n = (CH4N)2 = C2H8N2

9. Chapter 3 pg. 133 #’s 86, 87, 91 and 94 (only do the a’s)
Read 3.11 and 4.2 pgs and pgs