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Unit 7: The MOLE!
Chemistry 1
Counting by Weighing
1. A particular small laboratory cork weighs 1.63 g, whereas a rubber lab stopper of the same size weighs 4.31
g. How many corks would there be in 500. g of the laboratory corks? How many rubber stoppers would
there be in 500. g of similar lab stoppers? Let’s say you have 1.00 kg of corks. How many grams of rubber
stoppers would be needed to contain the same number as there are corks in the 1.00 kg of corks?
The Mole
2. Define the amu. What is one amu equivalent to in grams?
3. How is average atomic mass and molar mass related? What is the mass of 1 mole of hydrogen atoms?
4. In 24.02 g of carbon, there are _______________ carbon atoms.
5. What mass of calcium metal contains the same number of atoms as 12.16 g of magnesium? What mass of
calcium metal contains the same number of atoms as 24.31 g of magnesium?
6. Which weighs more, 0.50 mol of oxygen atoms or 4 mol of hydrogen atoms?
7. Using molar masses, calculate the number of moles of each element in samples with the following masses:
a. 26.2 g of gold
b. 41.5 g of calcium
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c. 335 mg of barium
d. 1.42 × 10-3 g of palladium
e. 3.05 × 10-5 µg of nickel
f. 12.01 g of carbon
8. Using molar masses, calculate the indicated quantities:
a. the number of cobalt atoms in 0.00103 g of cobalt
b. the number of cobalt atoms in 0.00103 mol of cobalt
c. the number of moles of cobalt represented by 5.99 × 1021 cobalt atoms
d. the mass of 4.23 mol of cobalt
e. the number of cobalt atoms in 4.23 g of cobalt
9. Calculate the molar mass for each of the following substances.
a. sodium nitride:
d. ethyl alcohol, C2H5OH
b. carbon disulfide:
e. sulfurous acid:
c. ammonium bromide, NH4Br
f. sulfuric acid:
10. Calculate the number of moles of the indicated substance in each of the following samples.
a. 49.2 mg of sulfur trioxide
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b. 7.44 × 104 kg of lead(IV) oxide
c. 59.1 g of chloroform, CHCl3
d. 3.27 µg of trichloroethane, C2H3Cl3
11. Calculate the mass in grams of each of the following samples.
a. .50 mol of aluminum iodide
b. 4.00 mol of glucose, C6H12O6
c. 4.56 × 105 mol of ethanol, C2H5OH
d. 2.27 mol of calcium nitrate
12. Calculate the number of molecules present in each of the following samples.
a. 6.37 mol of carbon monoxide
b. 6.37 g of carbon monoxide
c. 2.62 × 10-6 g of water
Percent Composition of Compounds
13. Calculate the percent by mass of each element in the following compounds.
a. sodium sulfate
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b. sodium sulfide
c. potassium phosphate
d. potassium hydrogen phosphate, K2HPO4
e. potassium phosphide
14. For each of the following ionic substances, calculate the percentage of the overall molar mass of the
compound that is represented by the positive ions the compound contains.
a. ammonium chloride
b. copper(II) sulfate, CuSO4
c. gold(III) chloride
d. silver nitrate
Formulas of Compounds
15. What experimental evidence about a new compound must be known before its formula can be determined?
16. What does the empirical formula of a compound represent? How does the molecular formula differ from
the empirical formula?
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17. Give the empirical formula that corresponds to each of the following molecular formulas.
a. sodium peroxide, Na2O2
b. terephthalic add, C8H6O4
c. phenobarbital, C12H12N2O3
d. 1,4-dichloro-2-butene, C4H6Cl2
18. Which of the following pairs of compounds have the same empirical formula?
a. acetylene, C2H2, and benzene, C6H6
b. ethane, C2H6,and butane, C4H10
c. NO2 and N2O4
d. C12H10O and C6H5OH
Calculation of Empirical Formulas
19. A new compound has been prepared. A 0.4791-g sample was analyzed and was found to contain the
following masses of elements: carbon, 0.1929 g; hydrogen, 0.01079 g; oxygen, 0.08566 g; chlorine,
0.1898g. Determine the empirical formula of the new compound.
20. In an experiment, a 2.514-g sample of calcium was heated in a stream of pure oxygen, and was found to
increase in mass by 1.004 g. Calculate the empirical formula of calcium oxide.
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21. A compound has the following percentages by mass: barium, 58.84%; sulfur, 13.74%; oxygen, 27.43%.
Determine the empirical formula of the compound.
22. If cobalt metal is mixed with excess sulfur and heated strongly, a sulfide is produced that contains 55.06%
cobalt by mass. Calculate the empirical formula of the sulfide.
23. If 2.461 g of metallic calcium is heated in a stream of chlorine gas, 4.353g of Cl2 is absorbed in forming the
metal chloride. Calculate the empirical formula of calcium chloride.
24. Sodium and nitrogen form two binary compounds. The percentages of the elements in these compounds
are:
Compound 1: 83.12% Na; 16.88% N
Compound 2: 35.36% Na; 64.64% N
Calculate the empirical formula of each of the compounds.
Compound 1:
Compound 2:
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Calculation of Molecular Formulas
25. How does the molecular formula of a compound differ from the empirical formula? Can a compound's
empirical and molecular formulas be the same? Explain.
26. A compound with the empirical formula CH2O found to have a molar mass between 89 and 91. What is the
molecular formula of the compound?
27. A compound having an approximate molar mass of 165-170 g has the following percentage composition by
mass: carbon, 42.87%; hydrogen, 3.598%; oxygen, 28.55%; nitrogen, 25.00%. Determine the empirical and
molecular formulas of the compound.
28. A compound consists of 65.45% C, 5.492% H, and 29.06% 0 on a mass basis and has a molar mass of
approximately 110. Determine the molecular formula of the compound.
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Challenge Problems
29. A binary compound of magnesium and nitrogen is analyzed, and 1.2791 g of the compound is found to
contain 0.9240 g of magnesium. When a second sample of this compound is treated with water and heated,
nitrogen is driven off as ammonia, leaving a compound that contains 60.31% magnesium and 39.69%
oxygen by mass. Calculate the empirical formulas of two magnesium compounds.
30. Find the item in column 2 that best explains or completes the statement or question in column 1.
Column 1
Column 2
(1) 1 amu
(a) 6.022 × 1023
(2) 1008 amu
(b) atomic mass
(3) mass of the "average" atom of an element
(c) mass of 1000 hydrogen atoms
(4) number of carbon atoms in 12.01 g of carbon
(d) benzene, C6H6 and acetylene,
(5) 6.022 × 1023 molecules
C 2H 2
(6) total mass of all atoms in 1 mol of a compound
(e) carbon dioxide
(7) smallest whole-number ratio of atoms present in a molecule
(f) empirical formula
(8) formula showing the actual number of atoms present in a molecule
(g) 1.66 × 10-24 g
(9) product formed when any carbon-containing compound is burned in O2 (h) molecular formula
(10) have the same empirical formulas, but different molecular formula
(i) molar mass
(j)1 mol
31. Calculate the number of grams of iron that contain the same number of atoms as 2.24 g of cobalt.
32. Calculate the number of grams of cobalt that contain the same number of atoms as 2.24 g of iron.
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33. A particular compound in the chemistry laboratory is found to contain 7.2 × 1024atoms of oxygen, 56.0 g of
nitrogen, and 4.0 mol of hydrogen. What is its empirical formula?
34. The compound A2O is 63.7% A (a mystery element) and 36.3% oxygen. What is the identity of element A?
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