1. Determining Empirical Formula

2. Empirical Formulas
Indicate the lowest whole number ratio of atoms of each element in a compound.
The molecular formula of hydrogen peroxide is H2O2 the empirical formula would be…? HO
Ho, Ho, Ho…

3. The molecular formula of glucose is…
C6H12O6, , its empirical formula is…
CH2O …
So what’s my point?
Empirical formulas are lowest whole number ratios of atoms of each type …to determine them experimentally we need numbers of atoms!

4. Consider the following:
Analysis of a compound at CSI North Haven Labs indicates that it contains
32.38 g Na
22.65 g S
g O
How can we use this data to determine the formula?...Think…
Mass  number of atoms!

5. Remember doing mole conversions?
Use the molar mass of element convert to moles.
A mole is a number of atoms … you now have a ratio of atoms in the compound.
Divide each mole amount by the smallest mole amount… whole # ratio???
Use ratio to write subscripts

6. 1.408 mol Na : mol S : 2.812mol O
Divide each by smallest 
1.993 mol Na : 1 mol S : mol O
Na2SO4

7. Try this..
Analysis indicates that a compound contains 78.1% Boron and 21.9% hydrogen determine the empirical formula.
Why is this one any different from the last example?
Assume you have a 100 gram sample…now how many grams of boron do you have?

8. 7.22 mol B : mol H
Divide through by smallest 
1 mol B : mol H
BH3

9. Try this one …
Analysis of a g sample of a compound known to contain only phosphorus and oxygen indicates a phosphorus content of g. What is the empirical formula?
Grams of P = g Grams of O = ?
Now try it…

10. mol P : mol O
Divide by smallest 
1 mol P : mol O …
Now what?...Multiply through by two to obtain whole numbers… 2 : 5
P2O5

11. Find empirical formulas…
36.48% Na, 25.41% S, % O…
% iron and 46.30% sulfur…
1.04 g K, 0.70 g Cr and 0.86 g O…
% C, 4.20% H and % O …

12. Quantitative Analysis
Various methods are used to determine the mass or percent composition of each element in a given compound…more about those later…