Q: A sample of a pure substance contains 0.50 moles and has a mass of 27.0 grams. What is the molar mass of the substance?

$$M = \frac{27.0\ \text{g}}{0.50\ \text{mol}} = 54\ \text{g/mol}$$

Q: A mass spectrum of a sample of chlorine shows two major peaks at mass numbers 35 and 37. What does this indicate about chlorine?

Chlorine exists as two isotopes, $$^{35}\text{Cl}$$ and $$^{37}\text{Cl}$$, with different abundances.

Q: A mixture contains 40% by mass of substance A (molar mass = 20 g/mol) and 60% by mass of substance B (molar mass = 30 g/mol). What is the average molar mass of the mixture?

$$\text{Average molar mass} = (0.40 \times 20) + (0.60 \times 30) = 8 + 18 = 26\ \text{g/mol}$$

Question 1

Which of the following best explains why the atomic radius decreases across a period from left to right in the periodic table?

Accepted Answer

The number of protons increases, increasing the effective nuclear charge, which pulls electrons closer to the nucleus.

Question 2

A sample of a pure substance contains 0.50 moles and has a mass of 27.0 grams. What is the molar mass of the substance?

Accepted Answer

$$M = \frac{27.0\ 	ext{g}}{0.50\ 	ext{mol}} = 54\ 	ext{g/mol}$$

Question 3

Which of the following statements about ionic compounds is correct?

Accepted Answer

Ionic compounds are formed when electrons are transferred from a metal to a nonmetal, resulting in oppositely charged ions that are held together by electrostatic forces.

Question 4

A mass spectrum of a sample of chlorine shows two major peaks at mass numbers 35 and 37. What does this indicate about chlorine?

Accepted Answer

Chlorine exists as two isotopes, $$^{35}	ext{Cl}$$ and $$^{37}	ext{Cl}$$, with different abundances.

Question 5

Which of the following best describes the process of photoelectron spectroscopy (PES)?

Accepted Answer

PES measures the energy required to remove electrons from atoms, providing information about electron configuration and energy levels.

Question 6

A mixture contains 40% by mass of substance A (molar mass = 20 g/mol) and 60% by mass of substance B (molar mass = 30 g/mol). What is the average molar mass of the mixture?

Accepted Answer

$$	ext{Average molar mass} = (0.40 	imes 20) + (0.60 	imes 30) = 8 + 18 = 26\ 	ext{g/mol}$$

Question 7

Which of the following electron configurations represents a ground state atom of magnesium (Mg, atomic number 12)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$\ $$3s^2$$

Question 8

Which of the following best explains why ionization energy generally increases across a period?

Accepted Answer

As the effective nuclear charge increases, electrons are held more tightly, requiring more energy to remove an electron.

Question 9

A student is given a sample of an unknown pure substance and determines its percent composition by mass to be 40% carbon, 6.7% hydrogen, and 53.3% oxygen. What is the empirical formula of the compound?

Accepted Answer

$$	ext{CH}_2	ext{O}$$

Question 10

Which of the following statements about valence electrons is correct?

Accepted Answer

Valence electrons are the outermost electrons and are primarily responsible for chemical bonding.

Question 11

Which of the following best describes the trend in electronegativity across a period in the periodic table?

Accepted Answer

Electronegativity increases from left to right across a period.

Question 12

A chemist needs to separate a mixture of two liquids with different boiling points. Which technique should be used?

Accepted Answer

Distillation

Question 13

Which of the following is the correct formula for calculating the number of moles ($$n) $$from mass ($$m) $$and molar mass ($$M$$)?

Accepted Answer

$$n = \frac{m}{M}$$

Question 14

Which of the following best explains why the first ionization energy of sodium is much lower than that of magnesium?

Accepted Answer

Sodium has one valence electron in the 3s orbital, which is more easily removed than magnesium's two valence electrons in the same orbital due to lower effective nuclear charge.

Question 15

A sample of a compound contains 12.0 g of carbon and 32.0 g of oxygen. What is the empirical formula of the compound?

Accepted Answer

$$	ext{CO}_2$$

Question 16

Which of the following is a correct statement about isotopes?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 17

Which of the following best describes the relationship between atomic number and the identity of an element?

Accepted Answer

The atomic number is equal to the number of protons in the nucleus and determines the identity of the element.

Question 18

Which of the following is the best explanation for the trend in atomic radius down a group in the periodic table?

Accepted Answer

Atomic radius increases down a group because additional energy levels are added, increasing the distance between the nucleus and the outermost electrons.

AP Chemistry Course Framework: Essential Study Notes for General Chemistry

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