Q: Which formula is used to calculate the average atomic mass of an element with multiple isotopes?

$$\text{Average atomic mass} = \sum (\text{fractional abundance} \times \text{isotope mass})$$

Question 1

Which scientist is credited with the discovery of the law of conservation of mass, and what does this law state?

Accepted Answer

Antoine Lavoisier; mass is neither created nor destroyed in a chemical reaction.

Question 2

According to Dalton's atomic theory, which statement is now known to be incorrect?

Accepted Answer

Atoms are indestructible and retain their identity in all chemical reactions.

Question 3

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

Accepted Answer

Rutherford's gold foil experiment

Question 4

What is the charge of an atom with 17 protons and 18 electrons?

Accepted Answer

$$-1$$

Question 5

Which of the following best describes isotopes?

Accepted Answer

Atoms of the same element with different numbers of neutrons

Question 6

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

Accepted Answer

$$28.09\ \mathrm{u}$$

Question 7

Which of the following is true about the atomic number ($$Z) $$and mass number ($$A) of an $$atom?

Accepted Answer

Atomic number ($$Z) is $$the number of protons; mass number ($$A) is $$the number of protons plus neutrons.

Question 8

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

Accepted Answer

$$18$$

Question 9

Which real-world application uses isotope ratios to determine the year of birth from tooth enamel?

Accepted Answer

Forensic science

Question 10

If the average atomic mass of gallium is $$69.723\ \mathrm{u}$$, and the masses of its isotopes are $$68.926\ \mathrm{u}$$ ($$^{69}\mathrm{Ga}) $$and $$70.925\ \mathrm{u}$$ ($$^{71}\mathrm{Ga}$$), which isotope is more abundant?

Accepted Answer

$$^{69}\mathrm{Ga}$$

Question 11

Which formula is used to calculate the average atomic mass of an element with multiple isotopes?

Accepted Answer

$$	ext{Average atomic mass} = \sum (	ext{fractional abundance} 	imes 	ext{isotope mass})$$

Question 12

Which subatomic particle determines the identity of an element?

Accepted Answer

Proton

Question 13

A mass spectrometer is used to measure:

Accepted Answer

The proportion of atoms belonging to each isotope in a sample

Question 14

Which of the following elements has only one naturally occurring isotope?

Accepted Answer

$$^{19}\mathrm{F}$$

Question 15

If an atom has 15 protons, 16 neutrons, and 15 electrons, what is its mass number?

Accepted Answer

$$31$$

Question 16

Which statement about the nucleus of an atom is correct?

Accepted Answer

It contains most of the atom's mass and is positively charged.

Question 17

Which of the following is a correct application of the law of definite proportions?

Accepted Answer

A sample of water always contains hydrogen and oxygen in a $$2:1$$ ratio by atom count.

Question 18

Which of the following would change the element identity of an atom?

Accepted Answer

Changing the number of protons

Question 19

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

Accepted Answer

Atoms of the same element can have different mass numbers.

Question 20

Which of the following best explains why atomic masses on the periodic table are not whole numbers?

Accepted Answer

They are weighted averages of all naturally occurring isotopes.

Atomic Structure and Nuclear Chemistry: Foundations and Applications

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