Chemistry 2014
Empirical Formulas Practice Test
Name: _____________________
*Check answer key online*
*Be sure and show all your work & units where calculations are required! No work = no points.
1. What is an empirical formula? The lowest whole number ratio of elements in a
compound. Give an example. CH2O
2. What is the percent composition of Oxygen in HClO3? 1 + 35 + (3 X 16) = 84 g/mol
H
Hydrogen
1.007947
1
1.19%
Cl
Chlorine
35.4532
1
41.98 %
O
Oxygen
15.99943
3
56.83 %
3. Write the empirical formula for a compound that contains 25.9 % N and 74.1 % oxygen.
Step 1: Find the moles
25.9 g N X 1 mole = 1.85 moles
74.1 g O X 1 mole = 4.63 moles
14 g
16 g
Step 2: Divide by the one with the least number of moles. The
smallest one will be one and the others will be multiples of that.
1.85/1.85 = 1
4.63/1.85 = 2.5
It can’t be NO2.5 If you multiply by 2, they are both
Whole numbers. N205
Answer 
Show work
N205
4. What is the empirical formula of a compound that contains 71.72 g chlorine, 16.16 g
oxygen and 12.2 g carbon?
Answer 
Show work
COCl2
What is the percent composition of Oxygen in this compound? 16/99 = 16.17%
Chemistry 2013
What is the empirical formula for a compound that is 33.88% Copper, 14.94% Nitrogen and
51.18% Oxygen. CuN2O6 = Cu(NO3)2
5.
Cu(NO3)2
Compound
Moles
Liters
Mass (g)
Particles
NO2
5.3
119
243.8
3.2 X 1024
CH4
40
90.86
649 g
2.4 X 1025
CO2
0.22
5
9.68
1.32 X 1023
6. If you dissolve 3 moles of NaCl in 1.5 liters of water, what is the molarity?
Molarity = Moles = 3 moles = 2 M
Liters
1.5 Liters
7. How many grams of NaCl would you need to make this solution?
3 moles of NaCl = 3 moles X 58 g = 174 g
1 mole
Chemistry 2013