1. Determining Empirical Formula from Combustion Analysis

2. Combustion Analysis CxHy + O2 → CO2 + H2O Combustion reaction:
Hydrocarbon + oxygen gas → carbon dioxide + water
CxHy + O2 → CO2 + H2O

3. Determining the Empirical Formula From Combustion Analysis Section 3
Determining the Empirical Formula From Combustion Analysis Section 3.5 (p. 97)
Another test to determine empirical formulas is the Combustion Analysis. The sample is combusted and the quantity of CO2 and H2O produced is measured.

4. Combustion Analysis
Example 1: Combustion of g of a hydrocarbon produces g CO2 and g H2O. What is the empirical formula?
All carbon from sample forms CO2
All hydrogen from sample forms H2O

5. Combustion Analysis
Example 1: Combustion of g of a hydrocarbon produces g CO2 and g H2O. What is the empirical formula?
CxHy + O2  CO2 + H2O

6. 0.250 g of compound X produces:
Combustion Analysis
Combustion Analysis: the technique of finding the mass composition of an unknown sample (X) by examining the products of its combustion.
X + O2 → CO2 + H2O
0.250 g of compound X produces:
0.686 g CO2 and g H2O

7. Combustion Analysis X + O2 → CO2 + H2O
Step 1. Find the mass of C & H that must have been present in X (multiply masses of products by percent composition of the products).
C: g CO2 x (1 mole CO2 /44.01 g CO2) X (1 mole C/1 mole CO2 ) X (12g C/1 mole C) = g C
H: g H2O x (1 mole H2O /18.02 g H2O) X (2 moles H/ 1 mole H2O) X (2.002 g H /2 moles of H) = g H

8. Combustion Analysis
X + O2 → CO2 + H2O
Find out if there’s oxygen or unknown compounds in X.
Subtract the mass of C and H from the original mass
0.250 g of compound X – (0.187 g C g H) = 0 g
So compound X must contain only C and H only and NO oxygen !!
Step 2. Find the number of moles of C and H
C: g x (1 mole C /12.01 g of C) = moles C
H: g x (1 mole H /1.008 g of H) = moles H

9. Combustion Analysis X + O2 → CO2 + H2O
3. Find the RELATIVE number of moles of C and H in whole number units (divide by smallest number of moles)
C: moles of C / = 1
H: moles of H / = 4
NOTE: If these numbers are fractions, multiply each by the same whole number.

10. Combustion Analysis X + O2 → CO2 + H2O
3. Write the Empirical Formula (use the relative numbers as subscripts)
CH4

11. Combustion Analysis Summary Find the mass of C and H in the sample.
Find the actual number of moles of C and H in the sample.
Find the relative number of moles of C and H in whole numbers.
Write the empirical formula for the unknown compound.

12. Combustion Analysis
Example 2: Combustion of g isopropyl alcohol produces g CO2 and g H2O. What is the empirical formula?
CxHyOz + O2 → CO2 + H2O
All carbon from sample forms CO2
All hydrogen from sample forms H2O
Oxygen must be determined from mass balance
____ g C + ______ g H + ______ g O = g sample

13. Combustion Analysis
Example 2: Combustion of g isopropyl alcohol produces g CO2 and g H2O. What is the empirical formula?
CxHyOz + O2 → CO2 + H2O

14. Combustion Analysis
Example 2: Combustion of g isopropyl alcohol produces g CO2 and g H2O. What is the empirical formula?