A blast from the past Combustion: when fuel burns, water and carbon dioxide are the products. Sometimes is a hydrocarbon fuel (contains carbon and hydrogen) while other times there are other components to the fuel source. Combustion Analysis: the technique of finding the mass composition of an unknown sample (X) by examining the products of its combustion. 1 Ex: Methanol combusts in the presence of oxygen 2CH 3 OH + 3O 2  2 CO H 2 0 Ex: Methane combusts in the presence of oxygen CH 4 + 2O 2  CO H 2 0

2 Combustion Analysis Combustion Analysis: the technique of finding the mass composition of an unknown sample (X) by examining the products of its combustion. X + O 2 → CO 2 + H 2 O g of compound X produces: g CO 2 and g H 2 O

3 Combustion Analysis X + O 2 → CO 2 + H 2 O 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). = g C C: g CO 2 x (1 mole CO 2 /44.01 g CO 2 ) X (1 mole C/1 mole CO 2 ) X (12g C/1 mole C) = g C = g H H: g H 2 O x (1 mole H 2 O /18.02 g H 2 O) X (2 moles H/ 1 mole H 2 O) X (2.002 g H /2 moles of H) = g H

4 Combustion Analysis X + O 2 → CO 2 + H 2 O Find out if there’s oxygen or unknown compounds in X. Subtract the mass of C and H from the original mass 0 g 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 moles C C: g x (1 mole C /12.01 g of C) = moles C moles H H: g x (1 mole H /1.008 g of H) = moles H

5 Combustion Analysis X + O 2 → CO 2 + H 2 O 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.

6 Combustion Analysis X + O 2 → CO 2 + H 2 O 3. Write the Empirical Formula (use the relative numbers as subscripts) CH 4

7 Combustion Analysis Summary 1. Find the mass of C and H in the sample. 2. Find the actual number of moles of C and H in the sample. 3. Find the relative number of moles of C and H in whole numbers. 4. Write the empirical formula for the unknown compound.

8 Combustion Analysis NOTE! In step # 1 always check to see if the total mass of C and H adds up to the total mass of X combusted. If the combined mass of C and H is less than the mass of X, then the remainder is an unknown element (unless instructed otherwise). If a third element is known, calculate the mass of that element by subtraction (at the end of step 1), and include the element in the remaining steps.

9 Combustion Analysis Combustion Analysis provides the Empirical Formula. If a second technique provides the molecular weight, then the molecular formula may be deduced. 1. Calculate the empirical formula weight. 2. Find the number of “formula units or (n)” by dividing the known molecular weight by the formula weight. 3. Multiply the number of atoms in the empirical formula by the number of formula units.

10 Combustion Analysis The molecular weight of glucose is 180 g/mole and its empirical formula is CH 2 O. Deduce the molecular formula. 1. Formula weight for CH 2 O is g/mole 2. # of “formula units or (n)” = (180 g/mole)/ (30.03g/mole) = 6 3. (CH 2 O)n  where n=6 Molecular formula = C 6 H 12 O 6

Example 1 Combustion of grams of fuel containing C, H, & O produces 0.561g of CO 2 and 0.306g of H 2 0 Step 1: Find the grams of Carbon Step 2: Find the grams of Hydrogen Subtract the mass of C and H from the original mass = mass of oxygen Step 3: Convert grams  moles  etc… 11

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Example 2 What is the chemical that makes your shoes smell? It’s generic name is X-stink: a mixture of carbon, hydrogen and oxygen. We analyzed a sample of your shoes and found that a 0.225g sample has 0.512g CO 2 and g H 2 0. If the molecular mass of X-stink is 96.11g/mole, what is its molecular formula? What’s our plan?? 1. Find the mass of carbon 2. Find the mass of hydrogen 3. Find the mass of oxygen 4. Find the empirical formula  the molecular formula 13

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Complete on your own – bring to class 0.548g of fuel composed of carbon, hydrogen and nitrogen is completely combusted grams of water and g of carbon dioxide are produced. The molecular formulas mass is 79.01g/mole. What are the empirical and molecular formulas? 15