Chemistry 11
Review of Empirical Formula, Molecular Formula,
& Determining the Formulae of Hydrates
Chemist _____________________________________________
1. A substance is composed of 3.05% carbon, 0.26% hydrogen, and 96.69% iodine. What is
the empirical formula of this substance?
a. CHI3
c. C2HI7
e. C3H2I11
b. CH2I2
d. CH2I5
2. Melissa is given an unknown to analyze for his empirical formula lab. She finds that the
compound is 76.6% carbon, 6.4% hydrogen, and 17.0% oxygen. What is the empirical
formula of his compound?
a. CHO
c. C3H3O
e. C6H6O
d. C4H3O
b. CH2O
3. A certain hydrate of thallium chloride is 53.4% thallium and 27.8% chlorine by mass. The
rest is water. What is the empirical formula of this compound?
a. TlCl3●H2O
b. TlCl3●2H2O
c. TlCl4·●3H2O
d. TlCl3·●4H2O
e. More information is required to answer the question.
4. A compound contains 63.6% tungsten by mass. The rest is chlorine. What is the empirical
formula of this compound?
c. WCl5
e. WCl
a. W2Cl3
b. WCl7
d. WCl3
5. A certain compound contains 72.0% iron and 28.0% oxygen. What is the empirical formula
of this compound?
a. Fe3O2
c. Fe3O4
e. FeO
b. Fe2O3
d. FeO2
6. An unknown compound is found to have the following composition: 37.5% C, 12.5% H,
and 50.0% O. What is the empirical formula of this compound?
a. CH3O4
c. C3HO4
e. C4H10O5
b. CH4O
d. C3H2O4
7. A 4.00-g sample of a compound was found to contain 1.96 g of tin, 0.46 g of nitrogen, and
1.58 g of oxygen. What is the empirical formula of this compound?
a. SnNO
c. SnNO3
e. Sn(NO3)2
b. SnN2O
d. Sn(NO3)3
1
8. A 5.00-g sample of a compound is found to contain 3.52 g of indium. The rest is sulfur.
What is the empirical formula of this compound?
a. In2S3
c. In2S
e. InS3
b. In2S2
d. InS2
9. The empirical formula of an compound is found to be CHCl. Upon analysis, the molar
mass is found to be 291 g/mol. How many carbon atoms are in each molecule of the
compound?
a. 3
c. 5
e. 8
b. 4
d. 6
10. The empirical formula of an certain hydrocortisone compound is found to be C13H19O3(s).
Upon analysis, the molar mass of the compound is found to be 446.6 g/mol. How many
hydrogen atoms are in each molecule of hydrocortisone?
a. 48
c. 28
e. 17
b. 38
d. 19
11. Kelsey analyzed a compound and found that it contained 74% carbon, 8.7% hydrogen,
and 17.3% nitrogen. She also determined that its molar mass was 162 g/mol. What is the
molecular formula of the compound?
c. C5H7N
e. CH2N
a. C20H28N4
b. C10H14N2
d. C2H2N
12. What is the empirical formula of a compound with a molecular formula that is P4O10?
a. PO
b. PO2
c. P2O5
d. P4O10
e. P8O20
13. A compound is found to be 86% carbon and 14% hydrogen. If the molar mass is 70.1
g/mol, what is the value of n in the following equation?
n=
a. 1
b. 2
c. 3
d. 4
e. 5
14. A compound contains 85% Ag and 15% F by mass. If the molar mass is 127 g/mol, what is
the value of n in the following equation?
n=
a. 1
b. 2
c. 3
d. 4
e. 5
2
15. What is the empirical formula of a compound that contains 46.2% carbon, 5.2% hydrogen,
and fluorine?
a. C3H4F2
b. C5H10F
c. C6H8F4
d. C6H6F2
e. C6H3F5
16. A certain plant contains a compound that can be extracted and used in medicine. The
empirical formula of this compound is CH2OS. It has a molar mass between 185.0 g/mol
and 190.0 g/mol. What is its molecular formula?
a. C2H6O2S2
b. C3H6O3S3
c. C3H4O3S3
d. C2H4O2S2
e. CH2OS
17. A 2.00-g sample compound contains 1.25 g of bromine. The rest is oxygen. What is the
empirical formula of this compound?
a. BrO
c. Br2O
e. Br3O
b. BrO2
d. BrO3
18. A certain compound was found to contain 13.9% nitrogen, 47.4% oxygen and the rest
potassium. What is the empirical formula of this compound?
c. K2NO2
e. K2N2O3
a. KNO2
b. KNO3
d. K2NO3
19. A 44.0-g sample of the hydrate CuSO4·xH2O is found to contain 15.9 g of H2O. What is the
value of x?
a. 3
c. 5
e. 7
b. 4
d. 6
20. A 50.0-g sample of the hydrate Al2(SO4)3·xH2O(s) is found to contain 19.4 g of H2O. What is
the value of x?
a. 3
c. 9
e. 15
b. 6
d. 12
21. A hydrate of magnesium sulfate, commonly known as Epsom salts, can be mixed with hot
water for a soothing bath. In a 5.00-g sample of this hydrate, 2.44 g was found to be
MgSO4(s). If the formula of the hydrate is MgSO4·xH2O(s), what is the value of x?
a. 3
c. 7
e. 11
b. 5
d. 9
22. Alum is a hydrated form of potassium aluminum sulfate. In a 25.00-g sample of this
hydrate, 13.7 g was found to be KAl(SO4)2(s). If the formula of the hydrate is
KAl(SO4)2·xH2O(s), what is the value of x?
a. 1
c. 6
e. 12
b. 3
d. 9
3
23. Gypsum is mined in Windsor, NS. It has many uses, one of which is to make plaster of
Paris. In a 500-kg sample of gypsum, CaSO4·xH2O(s), 395 kg was found to be CaSO4(s).
What is the value of x?
a. 1
c. 3
e. 5
b. 2
d. 4
24. Hydrated cobalt(II) chloride can be used to determine how much moisture is in the air.
When paper soaked in this compound is in a damp environment, the paper is pink in
colour. In a dry environment, the paper is blue. In a 10.00-g sample of the hydrate, 5.46 g
was found to be CoCl2. If the formula of the hydrate is CoCl2·xH2O(s), what is the value of
x?
a. 2
c. 6
e. 10
b. 4
d. 8
25. An unknown compound is found to be 20.3% aluminum and 79.7% chlorine. What is the
empirical formula?
26. The percentage composition of an organic compound is found to be 84.1% carbon, 11.8%
hydrogen, and 4.2% oxygen. What is the empirical formula of the cholesterol?
27. The percentage composition of acetaminophen, a common pain reliever, is found to be
63.6% carbon, 6.0% hydrogen, 21.2% oxygen, and 9.3% nitrogen. If the molar mass of this
compound is 150 g/mol, what is the molecular formula?
28. The percentage composition of acetylsalicylic acid, a common pain reliever, is found to be
60.1% carbon, 4.5% hydrogen, and 35.6% oxygen. If the molar mass of this compound is
180 g/mol, what is the molecular formula?
29. In a 15.0 g sample of a hydrate of copper (II) sulfate, CuSO4·xH2O, 9.2 g is copper(II)
sulfate. What is the value of x?
4