Download Empirical Formula Worksheet

Name: ___________
Period: ___ Date: ______
Empirical Formula Worksheet
Empirical Formula – a formula showing the smallest whole-number mole ratio of elements in a compound.
Molecular Formula -- the actual whole-number mole ratio of elements in a compound.
Example: The formula CH2 is an empirical formula because the ratio of C:H is 1:2 (smallest whole# ratio)
Compounds C2H4, C3H6, C4H8, C5H10, etc. are molecular compounds. They have the same
empirical formula (CH2). Their molar masses differ from each other by a whole# ratio. So if you
know the empirical formula and the molar mass of an unknown compound, you can determine its
molecular formula.
How to find EMPIRICAL FORMULA from PERCENT COMPOSITION.
(Note that this is just the reverse of finding percent composition from formula from the previous worksheet)
1. Find the mass of each element by assuming 100 grams of the compound.
2. Convert the grams of each element to moles. If the mole numbers are very close to whole numbers
(no more than 10%, or less than 0.1 or more than .9) you may round to that number.
3. To find the simplest ratio, divide each element’s mole value by the smallest mole value. This will
ensure a something to 1 ratio.
4. If the mole ratios are NOT all whole numbers, you must multiply a whole number in order to
obtain a whole number ratio.
5. It is helpful to recognize decimal forms of common fractions and multiply by the denominator.
Ex. 2.49 is close to 2 1/2 so multiply by 2 to get 5.
USE ANOTHER SHEET OF PAPER TO ANSWER THE FOLLOWING QUESTIONS:
1. Give the decimal forms of the following common fractions:
a.
1
/5
b. 1/4
c. 1/3
d. 1/2
e. 2/3
f. 3/4
2. A 15.0g sample of a compound is found to contain 8.83g sodium and 6.17g sulfur. Calculate the empirical
formula of this compound.
3. Find the empirical formula of a compound that contains 53.70% iron and 46.30% sulfur.
4. Analysis of a 10.150g sample of a compound known to contain only phosphorus and oxygen indicates a
phosphorus content of a 4.433g. What is the empirical formula of this compound.
5. A compound is found to contain 36.48% Na, 25.41% S, and 38.11% O. Find its empirical formula.
6. A compound is found to contain 63.52% iron and 36.48% sulfur. Find its empirical formula.
7. Qualitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen.
Find the empirical formula of this compound.
8. Find the empirical formula of a compound found to contain 26.56% potassium, 35.41% chromium and the
remainder oxygen.
9. Analysis of a 20.0g sample of a compound containing only calcium and bromine indicates that 4.00g of
calcium are present. What is the empirical formula of the compound formed?
10. A 60.00g sample of tetraethyl lead, a gasoline additive, is found to contain 38.43g lead, 17.83g carbon,
and 3.74g hydrogen. Find its empirical formula.
11. a. A compound of carbon and hydrogen has the composition of 92.25% carbon and
7.75% hydrogen by mass. What is the empirical formula of this composition?
b. If the compound has a mass of 52.03 g/mole, what is the molecular formula of the compound?
12. The compound meythl butanoate smells like apples. Its percent composition is 58.8% C, 9.8% H,
and 31.4% O. If its gram molecular mass is 102 g/mole, what is its molecular formula?
13. Find the empirical formula for a compound which contains 32.8% chromium and 67.2%
chlorine.
14. What is the empirical formula for a compound which contains 67.1% zinc and the rest is
oxygen?
15. What is the empirical formula for a compound containing 11% hydrogen and 89% oxygen?