Q: Calculate the average atomic mass of silicon given the following isotopic abundances and masses: $$^{28}\mathrm{Si} (92.23$$%, 27.97693 amu), $$^{29}\mathrm{Si} (4.67$$%, 28.97649 amu), $$^{30}\mathrm{Si} (3.09$$%, 29.97377 amu).

$$0.9223 \times 27.97693 + 0.0467 \times 28.97649 + 0.0309 \times 29.97377 = 28.0855$$ amu

Q: A certain element has two naturally occurring isotopes: $$X-69 (60.11$$%, 68.9257 amu) and $$X-71 (39.89$$%, 70.9249 amu). What is the average atomic mass of element $$X$$?

$$0.6011 \times 68.9257 + 0.3989 \times 70.9249 = 69.723$$ amu

Question 1

Which of the following statements best describes a key modification to Dalton's Atomic Theory based on modern understanding?

Accepted Answer

Atoms of a given element can have different numbers of neutrons, resulting in isotopes.

Question 2

A sample of a compound contains 7.100 g of hydrogen sulfide (H₂S). If 0.419 g of hydrogen is obtained from this sample, how many grams of sulfur are present?

Accepted Answer

$$7.100 - 0.419 = 6.681 g$$

Question 3

Which experiment provided evidence for the existence of a small, dense, positively charged nucleus at the center of the atom?

Accepted Answer

Rutherford's gold foil experiment

Question 4

Which of the following is the correct symbol for an isotope of iodine used in thyroid therapy with a mass number of 131 and 70 neutrons?

Accepted Answer

$$^{131}\mathrm{I}$$

Question 5

Calculate the average atomic mass of silicon given the following isotopic abundances and masses: $$^{28}\mathrm{Si} (92.23$$%, 27.97693 amu), $$^{29}\mathrm{Si} (4.67$$%, 28.97649 amu), $$^{30}\mathrm{Si} (3.09$$%, 29.97377 amu).

Accepted Answer

$$0.9223 	imes 27.97693 + 0.0467 	imes 28.97649 + 0.0309 	imes 29.97377 = 28.0855$$ amu

Question 6

Which of the following best explains why elements in the same group of the periodic table have similar chemical properties?

Accepted Answer

They have the same number of valence electrons.

Question 7

What is the empirical formula for a compound containing 40.0% sulfur and 60.0% oxygen by mass?

Accepted Answer

$$\mathrm{SO}_3$$

Question 8

Which of the following is the correct name for the compound $$\mathrm{Fe}_2\mathrm{O}_3$$?

Accepted Answer

Iron(III) oxide

Question 9

A certain element has two naturally occurring isotopes: $$X-69 (60.11$$%, 68.9257 amu) and $$X-71 (39.89$$%, 70.9249 amu). What is the average atomic mass of element $$X$$?

Accepted Answer

$$0.6011 	imes 68.9257 + 0.3989 	imes 70.9249 = 69.723$$ amu

Question 10

Which of the following compounds is a molecular compound?

Accepted Answer

$$\mathrm{CO}_2$$

Question 11

What is the correct formula for ammonium sulfate?

Accepted Answer

$$\mathrm{(NH_4$)_2SO_4}$$

Question 12

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 13

Which of the following ions is most likely to be formed by an atom of magnesium?

Accepted Answer

$$\mathrm{Mg}^{2+}$$

Question 14

What is the systematic name for $$\mathrm{CuCl}_2$$?

Accepted Answer

Copper(II) chloride

Question 15

Which of the following is the correct formula for phosphoric acid?

Accepted Answer

$$\mathrm{H_3PO_4}$$

Question 16

Which of the following best describes a cation?

Accepted Answer

An atom or group of atoms that has lost one or more electrons and has a positive charge.

Question 17

Which of the following is the correct name for $$\mathrm{N_2O_5}$$?

Accepted Answer

Dinitrogen pentoxide

Question 18

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 19

Which of the following is the correct formula for sodium hydrogen carbonate (baking soda)?

Accepted Answer

$$\mathrm{$$NaHCO_3$$}$$

Question 20

Which of the following best describes the difference between an ionic and a molecular compound?

Accepted Answer

Ionic compounds are formed from metals and nonmetals by electron transfer, while molecular compounds are formed from nonmetals by electron sharing.

Atoms, Molecules, and Ions: Foundations of General Chemistry

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