Q: A chlorine atom has two naturally occurring isotopes: $^{35}$Cl (34.97 amu, 75.77%) and $^{37}$Cl (36.97 amu, 24.23%). What is the average atomic mass of chlorine?

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

Question 1

Which of the following best illustrates the Law of Conservation of Mass in a chemical reaction?

Accepted Answer

When 7.7 g of Na reacts with 11.9 g of Cl$_2$ to form 19.6 g of NaCl, the total mass before and after the reaction remains the same.

Question 2

According to the Law of Definite Proportions, which statement is correct about water (H$_2$O) from different sources?

Accepted Answer

All samples of water, regardless of their source, have the same mass ratio of hydrogen to oxygen.

Question 3

A sample of water is decomposed to yield 16.0 g of oxygen and 2.0 g of hydrogen. What is the mass ratio of oxygen to hydrogen in water?

Accepted Answer

$\frac{16.0\ \mathrm{g}\ O}{2.0\ \mathrm{g}\ H} = 8:1$

Question 4

Which of the following best describes the Law of Multiple Proportions?

Accepted Answer

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Question 5

Carbon dioxide (CO$_2$) has a mass ratio of oxygen to carbon of 2.67:1, while carbon monoxide (CO) has a ratio of 1.33:1. What is the ratio of these two mass ratios, and what law does this illustrate?

Accepted Answer

$\frac{2.67}{1.33} = 2$, illustrating the Law of Multiple Proportions.

Question 6

Which statement is NOT part of Dalton’s Atomic Theory?

Accepted Answer

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

Question 7

Which experiment led to the discovery of the electron and its charge-to-mass ratio?

Accepted Answer

J.J. Thomson’s cathode ray tube experiment.

Question 8

What is the charge and approximate mass (in amu) of a neutron?

Accepted Answer

Charge = 0, Mass = 1.00866 amu

Question 9

An atom of sodium (Na) has an atomic number of 11. How many protons and electrons are present in a neutral sodium atom?

Accepted Answer

11 protons and 11 electrons

Question 10

Which of the following statements about isotopes is correct?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 11

A chlorine atom has two naturally occurring isotopes: $^{35}$Cl (34.97 amu, 75.77%) and $^{37}$Cl (36.97 amu, 24.23%). What is the average atomic mass of chlorine?

Accepted Answer

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

Question 12

Which group in the periodic table contains the noble gases?

Accepted Answer

Group 8A

Question 13

Which of the following is a property of metals but NOT of nonmetals?

Accepted Answer

High electrical conductivity that decreases with increasing temperature

Question 14

How many atoms are present in 6.03 mol of lead (Pb)? (Avogadro’s number = $6.022 	imes 10^{23}$)

Accepted Answer

$6.03 	imes 6.022 	imes 10^{23} = 3.63 	imes 10^{24}$ atoms

Question 15

A pure platinum ring contains $3.02 	imes 10^{22}$ platinum atoms. How many moles of platinum atoms does it contain?

Accepted Answer

$\frac{3.02 	imes 10^{22}}{6.022 	imes 10^{23}} = 0.0501$ mol

Question 16

What is the mass in grams of 2.3 $	imes$ $10^{-2}$ mol of antimony (Sb)? (Molar mass of Sb = 121.7 g/mol)

Accepted Answer

$2.3 	imes 10^{-2} 	imes 121.7 = 2.8$ g

Question 17

How many moles of argon (Ar) are contained in 11.8 grams of argon? (Molar mass of Ar = 39.95 g/mol)

Accepted Answer

$\frac{11.8}{39.95} = 0.295$ mol

Question 18

Which of the following best describes the relationship between atomic number and the periodic table?

Accepted Answer

The atomic number increases from left to right across a period and from top to bottom down a group.

Question 19

Which of the following ions has 17 protons and 18 electrons?

Accepted Answer

Cl$^-$

Atoms and Elements: Fundamental Laws and Atomic Theory

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