Question 1

Which scientific observation provided the first direct evidence for the existence of atoms and molecules, as illustrated by the random motion of particles in a liquid?

Accepted Answer

The observation of Brownian motion in pollen grains suspended in water.

Question 2

Refer to the images of yellow particles in water. What do the random movements of these particles represent at the molecular level?

Accepted Answer

Collisions between water molecules and the suspended particles.

Question 3

Which ancient philosopher is most closely associated with the idea that matter is composed of small, indivisible particles called atoms?

Accepted Answer

Democritus

Question 4

According to the law of conservation of mass, which equation correctly represents a chemical reaction where 10 g of hydrogen reacts with 80 g of oxygen to form water?

Accepted Answer

$$10\ \mathrm{g}\ \mathrm{H}_2 + 80\ \mathrm{g}\ \mathrm{O}_2 = 90\ \mathrm{g}\ \mathrm{H}_2\mathrm{O}$$

Question 5

Which statement best describes the law of definite proportions?

Accepted Answer

A compound always contains the same elements in the same proportion by mass.

Question 6

Given three samples of water with masses of 18 g, 36 g, and 54 g, each containing hydrogen and oxygen in a mass ratio of 1:8, what law does this observation support?

Accepted Answer

Law of definite proportions

Question 7

If two compounds are formed from the same two elements, and the mass ratio of one element in the first compound is exactly double that in the second, which law does this illustrate?

Accepted Answer

Law of multiple proportions

Question 8

Calculate the mass ratio of nitrogen to oxygen in dinitrogen pentoxide ($$\mathrm{N}_2\mathrm{O}_5) $$using atomic masses: N = 14.01, O = 16.00.

Accepted Answer

$$\frac{28.02}{80.00}$$

Question 9

Which of the following is NOT a postulate of Dalton’s atomic theory?

Accepted Answer

Atoms of one element can change into atoms of another element during a chemical reaction.

Question 10

Which historical figure is depicted in the image sitting by the sea, contemplating the nature of matter and the existence of atoms?

Accepted Answer

Democritus

Question 11

A chemist analyzes three samples of ammonia ($$\mathrm{NH}_3) $$and finds the mass ratio of nitrogen to hydrogen is always 14:3. What does this observation demonstrate?

Accepted Answer

The law of definite proportions

Question 12

Which of the following best explains why Brownian motion was considered evidence for the existence of atoms?

Accepted Answer

The random movement of visible particles is caused by collisions with invisible molecules.

Question 13

Which law is illustrated when the mass of reactants equals the mass of products in a closed chemical reaction?

Accepted Answer

Law of conservation of mass

Question 14

Which of the following statements is consistent with the anti-atomistic perspective of Aristotle and Plato?

Accepted Answer

Matter is infinitely divisible and composed of four elements: earth, air, fire, and water.

Question 15

Which of the following best describes the difference between the law of definite proportions and the law of multiple proportions?

Accepted Answer

The law of definite proportions states that a compound always has the same ratio of elements, while the law of multiple proportions applies when two elements form more than one compound.

Question 16

If 12 g of carbon reacts with 32 g of oxygen to form carbon dioxide, what is the mass of carbon dioxide produced according to the law of conservation of mass?

Accepted Answer

$$44\ \mathrm{g}$$

Question 17

Which of the following is a real-world example of the law of multiple proportions?

Accepted Answer

Carbon forms both carbon monoxide ($$\mathrm{CO}) $$and carbon dioxide ($$\mathrm{CO}_2) $$with oxygen, with different mass ratios of oxygen to carbon.

Question 18

Which statement is NOT part of Dalton’s atomic theory?

Accepted Answer

Atoms can be subdivided into protons, neutrons, and electrons.

Question 19

Which law explains why the ratio of nitrogen to hydrogen in ammonia ($$\mathrm{NH}_3) is $$always 14:3, regardless of the sample size?

Accepted Answer

Law of definite proportions

Question 20

Which of the following best summarizes the main insight from Einstein and Perrin’s work on Brownian motion?

Accepted Answer

It provided experimental evidence for the existence of atoms and molecules.

Atoms and Elements: Foundations of Modern Atomic Theory

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