Q: A sample of silicon contains 92.23% $$^{28}Si (27.9769 u$$), 4.67% $$^{29}Si (28.9765 u$$), and 3.10% $$^{30}Si (29.9738 u). $$What is the average atomic mass of silicon? (Round to 4 decimal places.)

$$0.9223 \times 27.9769 + 0.0467 \times 28.9765 + 0.0310 \times 29.9738 = 28.0855 u$$

Question 1

Which of the following statements best describes the law of conservation of mass as discovered by Antoine Lavoisier?

Accepted Answer

The total mass of substances before and after a chemical reaction remains unchanged.

Question 2

Which experiment provided evidence that most of the atom is empty space and that the nucleus is very small and dense?

Accepted Answer

Rutherford's gold foil experiment

Question 3

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

Accepted Answer

18

Question 4

Which of the following is a correct statement about isotopes?

Accepted Answer

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

Question 5

Which of the following best explains why the atomic mass of chlorine listed on the periodic table is not a whole number?

Accepted Answer

It is a weighted average of the masses of all naturally occurring isotopes of chlorine.

Question 6

Which of the following statements about Dalton’s atomic theory is no longer considered accurate?

Accepted Answer

All atoms of a given element are identical in mass and in all other properties.

Question 7

A sample of silicon contains 92.23% $$^{28}Si (27.9769 u$$), 4.67% $$^{29}Si (28.9765 u$$), and 3.10% $$^{30}Si (29.9738 u). $$What is the average atomic mass of silicon? (Round to 4 decimal places.)

Accepted Answer

$$0.9223 	imes 27.9769 + 0.0467 	imes 28.9765 + 0.0310 	imes 29.9738 = 28.0855 u$$

Question 8

Which of the following best describes the charge of an atom?

Accepted Answer

The charge is equal to the number of protons minus the number of electrons.

Question 9

Gallium has two naturally occurring isotopes: $$^{69}Ga (68.9256 u) $$and $$^{71}Ga (70.9250 u). $$The average atomic mass of gallium is 69.723 u. Which isotope is more abundant and why?

Accepted Answer

$$^{69}Ga is $$more abundant because the average atomic mass is closer to its mass.

Question 10

Which of the following real-world applications relies on the measurement of isotope ratios?

Accepted Answer

Determining the year of birth from tooth enamel using $$^{14}C $$levels.

Question 11

A neutral atom has 15 protons, 16 neutrons, and 15 electrons. What is its atomic number and mass number?

Accepted Answer

Atomic number = 15, Mass number = 31

Question 12

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

Accepted Answer

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

Question 13

Which of the following best explains why changing the number of protons in an atom changes the element?

Accepted Answer

The atomic number, which defines the element, is equal to the number of protons.

Question 14

Which of the following is a correct definition of a nucleon?

Accepted Answer

A nucleon is a general term for a proton or neutron.

Question 15

If a neutral atom becomes an ion with a charge of +2, what has happened to its subatomic particles?

Accepted Answer

It has lost 2 electrons.

Question 16

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

Accepted Answer

They have different numbers of neutrons.

Question 17

A forensic scientist uses mass spectrometry to analyze a sample. What information does the mass spectrum provide?

Accepted Answer

The proportion of atoms in the sample belonging to each isotope.

Atomic Structure and Nuclear Chemistry: Foundations and Applications

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