Q: Which of the following is the correct formula for calculating the average atomic mass (atomic weight) of an element?

$ \sum \text{(isotope mass)} \times \text{(fractional natural abundance)} $

Question 1

Which of the following statements best describes the Law of Constant Composition?

Accepted Answer

A compound always contains the same relative number and type of atoms, regardless of the sample source.

Question 2

Dalton’s atomic theory helped explain which of the following fundamental laws? Select all that apply.

Accepted Answer

Law of Conservation of Mass

Question 3

Which experiment provided the first direct evidence that atoms are made of smaller subatomic particles?

Accepted Answer

Thomson’s cathode ray tube experiment

Question 4

A neutral atom of sodium-23 ($ ^{23}\mathrm{Na} $) contains how many protons, neutrons, and electrons?

Accepted Answer

11 protons, 12 neutrons, 11 electrons

Question 5

Which of the following best explains why isotopes of the same element have different masses?

Accepted Answer

They have different numbers of neutrons.

Question 6

The charge-to-mass ratio ($ \frac{e}{m} $) of the electron was first measured by:

Accepted Answer

J.J. Thomson

Question 7

Which of the following is a correct application of the Law of Multiple Proportions?

Accepted Answer

The ratio of oxygen to carbon in CO and CO$_2$ is 1:2, a small whole-number ratio.

Question 8

A certain atom has 8 protons, 10 electrons, and 9 neutrons. What is its complete chemical symbol?

Accepted Answer

$ ^{17}_{8}\mathrm{O}^{2-} $

Question 9

Which of the following statements about the nucleus of an atom is correct?

Accepted Answer

The nucleus is about 100,000 times smaller than the atom.

Question 10

If the atomic number ($ Z $) of an element is 15 and its mass number ($ A $) is 31, how many neutrons does it have?

Accepted Answer

16

Question 11

Which of the following best describes the result of Rutherford’s gold foil experiment?

Accepted Answer

Most alpha particles passed straight through, but a few were deflected at large angles, indicating a small, dense, positively charged nucleus.

Question 12

Which of the following is the correct formula for calculating the average atomic mass (atomic weight) of an element?

Accepted Answer

$ \sum 	ext{(isotope mass)} 	imes 	ext{(fractional natural abundance)} $

Question 13

A sample of chlorine contains 75% $ ^{35}\mathrm{Cl} $ (mass = 35 amu) and 25% $ ^{37}\mathrm{Cl} $ (mass = 37 amu). What is the average atomic mass of chlorine?

Accepted Answer

35.5 amu

Question 14

Which of the following statements about subatomic particles is true?

Accepted Answer

Protons and neutrons have nearly the same mass, while electrons have much less mass.

Question 15

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

Accepted Answer

Atoms of one element can be converted into atoms of another element by chemical reactions.

Atoms, Molecules, and Ions – Chapter 2 Study Notes

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