Empirical and Molecular Formulas

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BRADY PROBLEM 2.83
Tutorial on the procedure of solving
for empirical and molecular formulas
from a combustion reaction.
Problem
Citric Acid, the substance that makes lemon juice
sour, is composed of only carbon , hydrogen, and
oxygen. When a 0.5000 g sample of citric acid was
burned, it produced 0.6871 g CO2 and 0.1874 g H2O.
(C + H + O)
0.5000 g
+
O2
CO2
0.6871 g
+
H2O
0.1874 g
The molecular mass of the compound is 192. What
are the empirical and molecular formulas for citric
acid?
All the hydrogen has gone to the product of H2O
All the carbon has gone to the product of CO 2
(C + H + O)
0.5000 g
+
O2
CO2
0.6871 g
+
H2 O
0.1874 g
From these gram quantities, convert to moles of compound,
then to moles of elements…(you may wish to subtotal as you
will use the number of moles later)…then to grams of
elements.
Starting with CO2….
Convert grams of CO2 to moles of CO2
to moles of C then to grams of carbon.
0.6871 g CO2 1 mol CO2
44.0 g
= 0.01562 mol C
1 mol C
= 0.01562 mol C
1 mol CO2
12.011 g
1 mol C
= 0.1876 grams C
Convert grams of H2O to moles of H2O
to moles of H then to grams of hydrogen.
0.1874 g H2O 1 mol H2O
18.0 g
= 0.02082 mol H
2 mol H
= 0.02082 mol H
1 mol H2O
1.008 g
1 mol H
= 0.02099 grams H
Find the number of grams of oxygen
in the original sample
0.5000 g
<0.02099 g>
<0.1876 g>
0.2914 g
original sample size
subtract grams hydrogen
subtract grams carbon
remaining grams are the grams of
oxygen in the original sample
Convert these grams to moles of oxygen
0.2914 g O
1 mol O
15.994 g
= 0.01821 mol O
Find the lowest mole ratio for the
empirical formula
x 6 =
6
0.02082 mol H/ 0.1562 = 1.333
x 6 =
8
0.01821 mol O/ 0.1562 = 1.166
x 6 =
7
0.1562 mol C/ 0.1562 =
1
C 6H 8O 7
Find the Molecular Formula
from the given Molecular Weight
Given molecular weight = 192 g/mol
Empirical Formula determined: C6H7O7
Calculate the mass of the empirical formula:
C = 12.011 X 6 = 72.061
H = 1.008 x 8 =
8.064
O = 15.9994 x 7 = 111.99
192.12 g/mol
Divide the mass of the empirical into the molecular weight:
192/192.12 = 0.99938 = 1
therefore the empirical formula is
the same as the molecular formula
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