Download Model—Empirical Formula Determination Problem—A compound

Model—Empirical Formula Determination
1.
Problem—A compound found by analysis to contain 75% carbon and 25% hydrogen. What is
the empirical formula?
Follow these steps:
1. C- 75% = 75 g
H- 25% = 25g
2. C- 75 gC * 1 moleC = 6.25 molesC
12 gC
H—25 gH * 1 moleH = 25 molesH
1 gH
3. Mole Ratio-(do exactly as follows)—
C:H = 6.25 mole: 25 mole (read as: C is to H as 6.25 is to 25).
4. Reduce Ratio—(divide by smallest number)—
C:H = 6.25 mole : 25 mole = 1 : 4
6.25 mole 6.25 mole
5. CH4
2.
Orlon is a synthetic material that is used in making clothing. It consists of 67.9% C, 5.7% H, and
26.4% N. Write the empirical formula.
Solution:
C-67.9% = 67.9 gC * 1 moleC = 5.66 moleC
12 gC
H-5.7% = 5.7 gH * 1 moleH = 5.7 moleH
1 gH
N-26.4% = 26.4 gN * 1 moleN = 1.89 moleN
14 gN
Mole Ratio
C: H: N = 5.66 mole : 5.7 mole: 1.89 mole = 3: 3: 1
1.89 mole 1.89 mole 1.89 mole
C3H3N Empirical Formula for Orlon
3. Propane gas is used as a fuel in camp stoves and lanterns. It consists of 81.7% C and 18.3% H.
Determine its empirical formula.
Solution:
C—81.7% = 81.7 g * 1 moleC = 6.81 moleC
12 gC
H—18.3% = 18.3 gH * 1 moleH = 18.3 moleH
1 gH
Mole Ratio
C: H = 6.81 mole : 18.3 mole = 1 : 2.69
6.81 mole 6.81 mole
These are not whole
numbers so we need
to adjust to obtain a
whole number ratio.
For our purposes we round off to the nearest 1/4, 1/3, 1/2, 2/3, 3/4, etc. or whole number.
So C: H = 1: 2.69 becomes C: H = 1 : 2 and 2/3
Now we convert 2 and 2/3 to an improper fraction.
We get C: H = 1: 8/3.
Now we can clear the fractions by multiplying by 3.
Empirical formula for propane gas is C3H8
C: H = 3: 8