AP Chemistry : Identifying and Defining Acids and Bases

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #1 : Identifying And Defining Acids And Bases

Which of the following is a Lewis base?

Possible Answers:

Me3B

B2H6

BF3

NH3

Correct answer:

NH3

Explanation:

NH3 should be the clear correct choice, since it is the only one having a pair of electrons that are available. The rest of the answer choices are all Lewis acids.

Example Question #1 : Identifying And Defining Acids And Bases

What is the defining characteristic of Bronsted-Lowry bases?

Possible Answers:

Dissociates in solution to give the H+ ion

Dissociates in solution to give the OH– ion

Proton (H+) donor

Electron pair acceptor

Proton (H+) acceptor

Correct answer:

Proton (H+) acceptor

Explanation:

The definition of a Bronsted-Lowry base is a species that has the ability to gain, or accept a proton (H+). Dissociating in solution is part of the Arrhenius definition of acids and bases, and Lewis acid are electron pair acceptors. 

Example Question #51 : Reaction Types

Which of the following can act as a Lewis base?

Possible Answers:

HF

BF3

NH2NH2

CH3COOH

Correct answer:

NH2NH2

Explanation:

A Lewis base is an electron-pair donor. N atoms have a valence of 5, and in the NH2NH2 compound, it is only bonded to another N atom and 2 H, so it's only using 3 of its 5 valence electrons to form these bonds. Thus, each nitrogen has a pair of unbonded electrons and can act as a Lewis base.

Example Question #4 : Identifying And Defining Acids And Bases

In the following equation, which is the conjugate base of HClO4?

HClO4 + H2O → ClO4– + H3O+

Possible Answers:

H2O

none of the above

HClO4

H3O+

ClO4–

Correct answer:

ClO4–

Explanation:

The conjugate base of an acid will be the same compound, short one H atom. ClO4– is the only one that meets this criterion.

Example Question #5 : Identifying And Defining Acids And Bases

Which of the following is a Lewis base?

Possible Answers:

NH4+

PH3

CH2COOH

CH3CH2CH2CH3

Correct answer:

PH3

Explanation:

A Lewis base is an electron-pair donor. Only PH3 has a pair of nonbonding electrons and can act as a donor. 

Example Question #6 : Identifying And Defining Acids And Bases

Which of the following is the conjugate base of oxalic acid (H2C2O4)?

Possible Answers:

HC2O4–

HC2O4+

H2C2O4

(C2O4) 2–

Correct answer:

HC2O4–

Explanation:

The conjugate base has one less H atoms and one unit greater negative charge because of this. Thus, the correct answer is HC2O4–

Example Question #7 : Identifying And Defining Acids And Bases

Which of the following solutions will have a pH greater than 7?

Possible Answers:

Correct answer:

Explanation:

HCN, HCl, CH3COOH, and NH4Cl are all acids (NH4+ is the ammonium ion). That only leaves KCN as the correct answer.

Example Question #8 : Identifying And Defining Acids And Bases

BCl3 is a                     .  

Possible Answers:

Arrhenius acid

Lewis acid

Lewis base

Bronsted-Lowry acid

Bronsted-Lowry base

Correct answer:

Lewis acid

Explanation:

A Lewis acid is a species that can accept an electron pair. In the BCl3 molecule, B does not have a complete octet (3 covalent bonds, thus 6 electrons around it rather than 8). Thus, it can accept another electron pair, making it a Lewis acid.

Example Question #9 : Identifying And Defining Acids And Bases

Which of the following compounds is a Lewis acid: HCl, AlCl_3, NH_3, H_2SO_4?

Possible Answers:

H_2SO_4

AlCl_3

HCl

H_2O

NH_3

Correct answer:

AlCl_3

Explanation:

A Lewis acid is a 2-electron acceptor, and a Lewis base is a 2-electron donor.

HCl, while it is acidic, cannot accept two electrons, so it is not a Lewis acid.

AlCl_3 can accept two electrons, so it is a Lewis acid.

NH_3 is a Lewis base.

H_2SO_4, like hydrochloric acid, is acidic, but cannot accept two electrons, so it is not a Lewis acid.

Example Question #10 : Identifying And Defining Acids And Bases

Which of the following salts will result in an acidic solution?

Possible Answers:

Correct answer:

Explanation:

All of the listed salts will dissolve into ions when in water. When the ions are in solution, they can act as acids or bases by donating or accepting protons. Chloride, bromide, and iodide ions are all conjugate bases of strong acids, so they will not accept protons. Sodium and potassium ions are the conjugate acids of strong bases, which dissociate completely, so they will not accept hydroxide ions.

Ammonium is the conjugate acid of ammonia, a weak base. The ammonium ion can donate a proton to the solution. This will make the solution slightly acidic. As a result, ammonium bromide is a salt that will make an acidic solution.

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