Q: A sample of naturally occurring chlorine consists of 75.77% $$^{35}\mathrm{Cl} ($$mass = 34.97 amu) and 24.23% $$^{37}\mathrm{Cl} ($$mass = 36.97 amu). What is the atomic mass of chlorine?

$$0.7577 \times 34.97 + 0.2423 \times 36.97 = 35.45$$ amu

Q: How many atoms are present in 2.00 moles of helium?

$$2.00 \times 6.022 \times 10$$^{23}$$ = 1.204 \times 10$$^{24}$$ atoms

Q: A 67.2-g sample of a gold (Au) and palladium (Pd) alloy contains $$2.49 \times 10$$^{23}$$ atoms. If the alloy is 25% Au by mass, what is the mass of gold in the sample?

$$0.25 \times 67.2\ \mathrm{g} = 16.8\ \mathrm{g}$$

Question 1

Which law states that in a chemical reaction, the total mass of reactants equals the total mass of products, as illustrated by the burning of wood in a closed system?

Accepted Answer

Law of Conservation of Mass

Question 2

A sample of water is decomposed into 16.0 g of oxygen and 2.0 g of hydrogen. What is the mass ratio of oxygen to hydrogen in water, and which law does this demonstrate?

Accepted Answer

$$\frac{16.0\ \mathrm{g}\ O}{2.0\ \mathrm{g}\ H} = 8:1$$; Law of Definite Proportions

Question 3

If 1.33 g of oxygen combines with 1 g of carbon to form carbon monoxide, and 2.67 g of oxygen combines with 1 g of carbon to form carbon dioxide, what is the ratio of the masses of oxygen that combine with a fixed mass of carbon? Which law does this illustrate?

Accepted Answer

$$\frac{2.67}{1.33} = 2:1$$; Law of Multiple Proportions

Question 4

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

Accepted Answer

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

Question 5

Which subatomic particle has a mass of approximately $$1.67 	imes 10$$^{-27}$$ kg and a relative charge of +1?

Accepted Answer

Proton

Question 6

A neutral atom of chlorine has an atomic number of 17 and a mass number of 35. How many protons, neutrons, and electrons does it have?

Accepted Answer

17 protons, 18 neutrons, 17 electrons

Question 7

Which isotope notation correctly represents an atom with 10 protons and 12 neutrons?

Accepted Answer

$$^{22}_{10}\mathrm{Ne}$$

Question 8

A sample of naturally occurring chlorine consists of 75.77% $$^{35}\mathrm{Cl} ($$mass = 34.97 amu) and 24.23% $$^{37}\mathrm{Cl} ($$mass = 36.97 amu). What is the atomic mass of chlorine?

Accepted Answer

$$0.7577 	imes 34.97 + 0.2423 	imes 36.97 = 35.45$$ amu

Question 9

Which of the following best describes a cation?

Accepted Answer

An atom that has lost one or more electrons

Question 10

A main-group nonmetal tends to form an anion with the same number of electrons as the nearest noble gas. Which of the following ions is isoelectronic with neon ($$\mathrm{Ne}$$)?

Accepted Answer

$$\mathrm{F}^-$$

Question 11

Which scientist is credited with the discovery of the periodic law and the first periodic table?

Accepted Answer

Dmitri Mendeleev

Question 12

Which of the following statements about the modern periodic table is TRUE?

Accepted Answer

Elements are arranged in order of increasing atomic number.

Question 13

A smoke detector uses the isotope $$^{241}\mathrm{Am}. $$What is the role of this isotope in the device?

Accepted Answer

It emits alpha particles that ionize air, allowing current to flow.

Question 14

How many atoms are present in 2.00 moles of helium?

Accepted Answer

$$2.00 	imes 6.022 	imes 10$$^{23}$$ = 1.204 	imes 10$$^{24}$$ atoms

Question 15

What is the molar mass of carbon dioxide ($$\mathrm{CO}_2$$)?

Accepted Answer

$$44.01\ \mathrm{g/mol}$$

Question 16

A 67.2-g sample of a gold (Au) and palladium (Pd) alloy contains $$2.49 	imes 10$$^{23}$$ atoms. If the alloy is 25% Au by mass, what is the mass of gold in the sample?

Accepted Answer

$$0.25 	imes 67.2\ \mathrm{g} = 16.8\ \mathrm{g}$$

Question 17

Which of the following best describes the difference between isotopes of the same element?

Accepted Answer

They have the same number of protons but different numbers of neutrons.

Question 18

Which of the following elements is a main-group metal?

Accepted Answer

Sodium (Na)

Question 19

A sealed jar contains a volatile liquid. After the liquid vaporizes, what happens to the mass of the sealed jar and its contents?

Accepted Answer

The mass remains unchanged.

Question 20

Which of the following is the correct conceptual plan for converting grams of an element to number of atoms?

Accepted Answer

$$	ext{grams} ightarrow 	ext{moles} ightarrow 	ext{atoms}$$

CHEM 131 Chapter 2: Atoms and Elements: Foundations of Modern Atomic Theory

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