Chapter 3 practice problems
Use your notes, book and help from classmates to work out the following conservation of mass, law of
definite proportions and law of multiple proportions problems.
1.
106.5g of hydrochloride (HCl) gas reacts with an unknown amount of ammonia, NH3, to
produce 157.5g of ammonium chloride, NH4Cl. How many grams of ammonia, NH3, reacted?
51.0g
2.
1.0g of hydrogen reacts completely with 19.0g of fluorine. What is the percent by mass of
hydrogen in the compound that is formed?
5% hydrogen
3. Two unknown compounds are tested. Compound I contains 15.0g of hydrogen and 120.0g of
oxygen. Compound II contains 2.0g of hydrogen and 32.0g of oxygen. Are the compounds the
same? Explain your answer.
No, percent by mass of is not the same in each compound:
Compound I: 11% hydrogen, 89% oxygen
Compound II: 5.8% hydrogen, 94.2% oxygen
4. Complete the table and describe the relationship between the two iron oxide compounds:
Compound
I
Total Mass
(g)
75.00
Mass Fe
(g)
52.46
Mass O
(g)
22.54
II
56.00
43.53
12.47
Mass
percent Fe
Mass
percent O
30.05%
Mass ratio
(Fe to O)
2.3:1
22.27%
3.48:1
69.95%
77.73%
How do the masses of iron in each compound compare to each other?
2.3 : 3.48
=
2:3
5.
A substance breaks down into its component elements when it is heated. If 68.0g of the
substance is present before it is heated, what is the combined mass of the component elements
after heating?
68.0g
6. Copper sulfide is formed when copper and sulfur are heated together. In this reaction, 127g of
copper reacts with 41g of sulfur. After the reaction is complete, 9g of sulfur remains unreacted.
What is the mass of copper sulfide formed?
159g
7. When burning 180g of glucose in the presence of 192g of oxygen, water and carbon dioxide are
produced. If 108g of water is produced, how much carbon dioxide is produced?
264g
8. A 25.3g sample of an unknown compound contains 0.8g of oxygen. What is the percent by mass
of oxygen in the compound?
3%
9. When mercury oxide is heated, it decomposes into mercury and oxygen. If 28.4 g of mercury
oxide decomposes, producing 2.0g of oxygen, what is the percent by mass of mercury in
mercury oxide?
93%
10. Which law would you use to compare CO and CO2? Which of the two compounds has the
highest percent by mass of oxygen in the compound?
Law of multiple proportions, CO2 will have double that in CO