Q: A naturally occurring element consists of two isotopes: 70% isotope A (mass = 10 amu) and 30% isotope B (mass = 12 amu). What is the atomic mass of the element?

$$0.70 \times 10 + 0.30 \times 12 = 10.6$$ amu

Q: How many atoms are present in 0.50 mol of aluminum?

$$0.50 \times 6.022 \times 10$$^{23}$$ = 3.01 \times 10$$^{23}$$ atoms

Q: A sample contains $$1.20 \times 10$$^{24}$$ atoms of copper. How many moles of copper are present?

$$\frac{1.20 \times 10$$^{24}$$}{6.022 \times 10$$^{23}$$} = 1.99$$ mol

Question 1

Which of the following statements best distinguishes between a scientific law and a scientific theory?

Accepted Answer

A law is a brief statement that summarizes past observations, while a theory provides the underlying reasons for those observations.

Question 2

A sample of a compound contains 12.0 g of carbon and 32.0 g of oxygen. According to the law of definite proportions, what is the mass ratio of oxygen to carbon in this compound?

Accepted Answer

$$\frac{32.0}{12.0} = 2.67$$

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

A naturally occurring element consists of two isotopes: 70% isotope A (mass = 10 amu) and 30% isotope B (mass = 12 amu). What is the atomic mass of the element?

Accepted Answer

$$0.70 	imes 10 + 0.30 	imes 12 = 10.6$$ amu

Question 5

Which of the following best describes a homogeneous mixture?

Accepted Answer

A mixture with uniform composition throughout, such as saltwater.

Question 6

Which statement about subatomic particles is correct?

Accepted Answer

Protons and neutrons have nearly identical masses, while electrons are much less massive.

Question 7

A neutral atom of magnesium has an atomic number of 12 and a mass number of 24. How many protons, neutrons, and electrons does it have?

Accepted Answer

12 protons, 12 neutrons, 12 electrons

Question 8

Which of the following is an example of the law of multiple proportions?

Accepted Answer

Carbon forms two oxides: CO (1.33 g O per 1 g C) and CO$$_2 (2.67 g O $$per 1 g C); the ratio of oxygen masses is 2:1.

Question 9

How many atoms are present in 0.50 mol of aluminum?

Accepted Answer

$$0.50 	imes 6.022 	imes 10$$^{23}$$ = 3.01 	imes 10$$^{23}$$ atoms

Question 10

A sample contains $$1.20 	imes 10$$^{24}$$ atoms of copper. How many moles of copper are present?

Accepted Answer

$$\frac{1.20 	imes 10$$^{24}$$}{6.022 	imes 10$$^{23}$$} = 1.99$$ mol

Question 11

Which of the following best describes an isotope?

Accepted Answer

Atoms of the same element with different numbers of neutrons.

Question 12

A sample of chlorine contains 75.77% $$^{35}Cl ($$mass = 34.97 amu) and 24.23% $$^{37}Cl ($$mass = 36.97 amu). Calculate the atomic mass of chlorine.

Accepted Answer

$$0.7577 	imes 34.97 + 0.2423 	imes 36.97 = 35.45$$ amu

Question 13

Which of the following statements about ions is correct?

Accepted Answer

Cations are positively charged ions formed when atoms lose electrons.

Question 14

Which of the following is a correct conversion factor for converting between grams and moles of carbon (atomic mass = 12.01 g/mol)?

Accepted Answer

$$\frac{1 	ext{ mol C}}{12.01 	ext{ g C}}$$

Question 15

A gold nugget has a mass of 19.3 g. How many moles of gold does it contain? (Molar mass of Au = 197.0 g/mol)

Accepted Answer

$$\frac{19.3}{197.0} = 0.098$$ mol

Question 16

Which of the following best describes the main result of Thomson's cathode ray experiment?

Accepted Answer

Atoms contain negatively charged particles called electrons.

Question 17

Which of the following is a real-world application of the law of conservation of mass?

Accepted Answer

The total mass of reactants equals the total mass of products in a sealed chemical reaction.

Question 18

A sample of pure silver contains $$2.50 	imes 10$$^{22}$$ atoms. What is the mass of the sample? (Molar mass of Ag = 107.87 g/mol)

Accepted Answer

$$\frac{2.50 	imes 10$$^{22}$$}{6.022 	imes 10$$^{23}$$} 	imes 107.87 = 4.48 g$$

Atoms and the Classification of Matter: General Chemistry Study Notes

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