Download PowerPoint Presentation - Formulas, Equations, and Moles

A blast from the past
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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.
Ex: Methanol combusts in the
presence of oxygen
2CH3OH + 3O2  2 CO2 + 4 H20
Ex: Methane combusts in the
presence of oxygen
CH4 + 2O2  CO2 + 2 H20
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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 0.562 g H2O
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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: 0.686 g CO2 x (1 mole CO2 /44.01 g CO2) X (1 mole C/1 mole CO2 ) X (12g C/1 mole C) = 0.187 g C
H: 0.562 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) = 0.063 g H
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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 + 0.063 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: 0.187 g x (1 mole C /12.01 g of C) = 0.0156 moles C
H: 0.063 g x (1 mole H /1.008 g of H) = 0.063 moles H
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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: 0.0156 moles of C /0.0156
=1
H: 0.063 moles of H /0.0156
=4
NOTE: If these numbers are fractions, multiply each by the same
whole number.
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Combustion Analysis
X + O2 → CO2 + H2O
3. Write the Empirical Formula (use the relative numbers as
subscripts)
CH4
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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.
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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.
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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.
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Combustion Analysis
The molecular weight of glucose is 180 g/mole and its
empirical formula is CH2O. Deduce the molecular
formula.
1.
Formula weight for CH2O is 30.03 g/mole
2.
# of “formula units or (n)” = (180 g/mole)/ (30.03g/mole) = 6
3.
(CH2O)n  where n=6
Molecular formula = C6H12O6
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Example 1
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Combustion of 0.255 grams of fuel containing C, H, & O
produces 0.561g of CO2 and 0.306g of H20
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Step 1: Find the grams of Carbon
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Step 2: Find the grams of Hydrogen
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Subtract the mass of C and H from the original mass = mass of
oxygen
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Step 3: Convert grams  moles  etc…
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Example 2
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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 CO2 and 0.209
g H20. If the molecular mass of X-stink is 96.11g/mole,
what is its molecular formula?
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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
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Complete on your own – bring to class
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0.548g of fuel composed of carbon, hydrogen and
nitrogen is completely combusted. 0.312 grams of
water and 1.525 g of carbon dioxide are produced.
The molecular formulas mass is 79.01g/mole. What
are the empirical and molecular formulas?
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