OpenStax Chemistry: Atoms First 2e
2.1: Early Ideas in Atomic Theory
Chemistry: Atoms First 2e
2: Atoms, Molecules, and Ions
2.1: Early Ideas in Atomic Theory
1. In the following drawing, the green spheres represent atoms of a certain element. The purple
spheres represent atoms of another element. If the spheres of different elements touch, they are
part of a single unit of a compound. The following chemical change represented by these spheres
may violate one of the ideas of Dalton’s atomic theory. Which one?
Solution
The starting materials consist of one green sphere and two purple spheres. The products consist
of two green spheres and two purple spheres. This violates Dalton’s postulate that that atoms are
not created during a chemical change, but are merely redistributed.
2. Which postulate of Dalton’s theory is consistent with the following observation concerning the
weights of reactants and products? When 100 grams of solid calcium carbonate is heated, 44
grams of carbon dioxide and 56 grams of calcium oxide are produced.
Solution
Atoms are neither created nor destroyed during a chemical change; they are rearranged. This
explains why the mass of the reactant equals the mass of the products.
3. Identify the postulate of Dalton’s theory that is violated by the following observations: 59.95%
of one sample of titanium dioxide is titanium; 60.10% of a different sample of titanium dioxide
is titanium.
Solution
This statement violates Dalton’s fourth postulate: In a given compound, the numbers of atoms of
each type (and thus also the percentage) always have the same ratio.
4. Samples of compound X, Y, and Z are analyzed, with results shown below.
Compound Description Mass of Carbon Mass of Hydrogen
X clear, colorless, liquid with strong odor 1.776 g 0.148 g
Y clear, colorless, liquid with strong odor 1.974 g 0.329 g
Z clear, colorless, liquid with strong odor 7.812 g 0.651 g
Do these data provide example(s) of the law of definite proportions, the law of multiple
proportions, neither, or both? What do these data tell you about compounds X, Y, and Z?
Solution
The carbon-to-hydrogen mass ratio in X is 12:1, in Y is 6:1, and in Z is 12:1. Compounds X and
Z have the same mass ratio, so they could be the same compound; that would be an example of
the law of definite proportions. The ratio of the mass ratios of X:Y as well as Z:Y are both 12:6 =
2:1, which are examples of the law of multiple proportions.
This resource file is copyright 2019, Rice University. All Rights Reserved.
Page 1 of 1
2.1: Early Ideas in Atomic Theory
Chemistry: Atoms First 2e
2: Atoms, Molecules, and Ions
2.1: Early Ideas in Atomic Theory
1. In the following drawing, the green spheres represent atoms of a certain element. The purple
spheres represent atoms of another element. If the spheres of different elements touch, they are
part of a single unit of a compound. The following chemical change represented by these spheres
may violate one of the ideas of Dalton’s atomic theory. Which one?
Solution
The starting materials consist of one green sphere and two purple spheres. The products consist
of two green spheres and two purple spheres. This violates Dalton’s postulate that that atoms are
not created during a chemical change, but are merely redistributed.
2. Which postulate of Dalton’s theory is consistent with the following observation concerning the
weights of reactants and products? When 100 grams of solid calcium carbonate is heated, 44
grams of carbon dioxide and 56 grams of calcium oxide are produced.
Solution
Atoms are neither created nor destroyed during a chemical change; they are rearranged. This
explains why the mass of the reactant equals the mass of the products.
3. Identify the postulate of Dalton’s theory that is violated by the following observations: 59.95%
of one sample of titanium dioxide is titanium; 60.10% of a different sample of titanium dioxide
is titanium.
Solution
This statement violates Dalton’s fourth postulate: In a given compound, the numbers of atoms of
each type (and thus also the percentage) always have the same ratio.
4. Samples of compound X, Y, and Z are analyzed, with results shown below.
Compound Description Mass of Carbon Mass of Hydrogen
X clear, colorless, liquid with strong odor 1.776 g 0.148 g
Y clear, colorless, liquid with strong odor 1.974 g 0.329 g
Z clear, colorless, liquid with strong odor 7.812 g 0.651 g
Do these data provide example(s) of the law of definite proportions, the law of multiple
proportions, neither, or both? What do these data tell you about compounds X, Y, and Z?
Solution
The carbon-to-hydrogen mass ratio in X is 12:1, in Y is 6:1, and in Z is 12:1. Compounds X and
Z have the same mass ratio, so they could be the same compound; that would be an example of
the law of definite proportions. The ratio of the mass ratios of X:Y as well as Z:Y are both 12:6 =
2:1, which are examples of the law of multiple proportions.
This resource file is copyright 2019, Rice University. All Rights Reserved.
Page 1 of 1
