Q: Which of the following is the correct formula for calculating the atomic mass of an element with multiple isotopes?

$$\text{Atomic mass} = \sum (\text{fraction of isotope}_n \times \text{mass of isotope}_n)$$

Q: How many atoms are in $$2.85 \times 10$$^{-3}$$ mol of gallium ($$\mathrm{Ga}$$)? Use Avogadro’s number 6.022$$ \times $$10^{23}.$$

$$1.72 \times 10$$^{21}$$ atoms

Question 1

Which experiment provided the first direct evidence for the existence of atoms as discrete particles?

Accepted Answer

Brownian motion observations and Einstein’s theoretical explanation

Question 2

Which law states that in a chemical reaction, matter is neither created nor destroyed?

Accepted Answer

Law of conservation of mass

Question 3

A sample of ammonia contains 10.0 g of hydrogen. If the mass ratio of nitrogen to hydrogen in ammonia is 4.7:1, how many grams of nitrogen are present?

Accepted Answer

$$47.0\ \mathrm{g}$$

Question 4

Which statement best distinguishes the law of definite proportions from the law of multiple proportions?

Accepted Answer

The law of definite proportions applies to two or more samples of the same compound, while the law of multiple proportions applies to two different compounds containing the same two elements.

Question 5

In the reaction $$7.7\ \mathrm{g} Na$$ + $$11.9\ \mathrm{g} Cl$_2$$ → $$19.6\ \mathrm{g}$$ NaCl, which law is demonstrated?

Accepted Answer

Law of conservation of mass

Question 6

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

Accepted Answer

Most alpha particles passed through, but some were deflected at large angles, indicating a dense nucleus.

Question 7

Which subatomic particle has a mass of approximately $$1.67 	imes 10$$^{-27}$$\ \mathrm{kg}$$ and a positive charge?

Accepted Answer

Proton

Question 8

A neutral atom of oxygen has 8 protons. How many electrons does the $$\mathrm{O}^{2-}$$ ion have?

Accepted Answer

$$10$$

Question 9

Which element contains 50 protons in its nucleus?

Accepted Answer

Tin

Question 10

An isotope of argon has a mass number of 40 and an atomic number of 18. How many neutrons does it contain?

Accepted Answer

$$22$$

Question 11

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 12

Which group of elements is characterized by being mostly unreactive and having a full valence shell?

Accepted Answer

Noble gases

Question 13

Which property is NOT typical of metals?

Accepted Answer

Tend to gain electrons in chemical reactions

Question 14

Which of the following best describes the periodic law?

Accepted Answer

When elements are arranged in order of increasing atomic number, certain sets of properties recur periodically.

Question 15

A main-group element in group 2A is most likely to form which ion?

Accepted Answer

$$2+$$ cation

Question 16

Which of the following is the correct formula for calculating the atomic mass of an element with multiple isotopes?

Accepted Answer

$$	ext{Atomic mass} = \sum (	ext{fraction of isotope}_n 	imes 	ext{mass of isotope}_n)$$

Question 17

How many atoms are in $$2.85 	imes 10$$^{-3}$$ mol of gallium ($$\mathrm{Ga}$$)? Use Avogadro’s number 6.022$$ 	imes $$10^{23}.$$

Accepted Answer

$$1.72 	imes 10$$^{21}$$ atoms

Question 18

Which experiment determined the charge of a single electron?

Accepted Answer

Millikan’s oil drop experiment

Question 19

Which of the following is true for all isotopes of a given element?

Accepted Answer

They have the same number of protons.

Question 20

Which of the following best describes a metalloid?

Accepted Answer

An element with properties intermediate between metals and nonmetals

Atoms and Elements: Foundations of Modern Chemistry

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