MCAT Physical : Defining/Classifying Acids and Bases

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #51 : Acid Base Chemistry

Which of the following is considered a weak base? 

Possible Answers:

Correct answer:

Explanation:

Strong bases are those that completely dissociate in water. All group I hydroxides are considered strong bases, including sodium hydroxide, potassium hydroxide, and lithium hydroxide. Some group II hydroxides are also strong bases, including calcium hydroxide, barium hydroxide, and strontium hydroxide.

Ammonia is considered a weak base and is frequently used as a buffer in solution with ammonium, its conjugate acid.

Example Question #52 : Acid Base Chemistry

Which of the following is not a strong acid? 

Possible Answers:

Nitric acid

Acetic acid

Hydrochloric acid

Perchloric acid

Correct answer:

Acetic acid

Explanation:

Strong acids are those that fully dissociate in an aqueous environment. Most protonated halogens are considered strong acids, with the exception of hydrogen fluoride; hydrochloric, hydrobromic, and hydroiodic acid are considered strong acids.

Perchloric acid is perhaps the strongest acid, and is formed from a hydrogen atom and a perchlorate ion: . Nitric acid is formed from a hydrogen atom and a nitrate ion, making it highly soluble and making it easy to dissolve in solution: .

Acetic acid, and most other organic acids, are considered weak acids since they lack stability when deprotonated. Stronger acids will form ions or resonace structures that are highly stable, favoring the release of the bound proton, while weaker acids will bind the proton more tightly to maintain a more stable structure. Acetic acid has the structure .

Example Question #53 : Acid Base Chemistry

Which of the following is a weak base?

 

Possible Answers:

Ba(OH)2

K2O

NaOH

NH3

Li2O

Correct answer:

NH3

Explanation:

Group 1 hydroxides, group 1 oxides, Ba(OH)2, Sr(OH)2, Ca(OH)2, and metal amides are the only strong bases. Everything else is considered a weak base. 

Example Question #54 : Acid Base Chemistry

Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.

All of the bases proceed in a similar fashion.

 

The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.

In the reverse reaction of , the proton is acting as a(n) __________, and is thus a __________.

Possible Answers:

electron acceptor . . . Lewis base

electron acceptor . . . Lewis acid

electron donor . . . Lewis base

electron donor . . . Brønsted-Lowry base

electron donor . . . Lewis acid

Correct answer:

electron acceptor . . . Lewis acid

Explanation:

In terms of the passage, the lone proton can be considered a proton donor and would, therefore, be a Brønsted-Lowry acid. This is not an answer choice.

The third acid-base definition is the Lewis definition, which states that acids are electron acceptors and bases are electron donors. The negative charge on the signifies that it is a Lewis base with available electrons to donate. The proton is accepting these electrons from , and is thus acting as a Lewis acid.

Example Question #55 : Acid Base Chemistry

The Haber-Bosch process, or simply the Haber process, is a common industrial reaction that generates ammonia from nitrogen and hydrogen gas. A worker in a company generates ammonia from the Haber process. He then dissociates the gaseous ammonia in water to produce an aqueous solution. Since ammonia is a base, it will accept a proton from water, generating  and ammonium ion products. The two reactions involved are:

What happens when ammonium ions are dissociated in water?

Possible Answers:

Water gains a proton and generates hydronium ions

Water gains a proton and generates hydroxide ions

Water loses a proton and generates hydroxide ions

Water is inert, so nothing happens

Correct answer:

Water gains a proton and generates hydronium ions

Explanation:

Ammonia is a base and produces ammonium ions in water; therefore, ammonium is the conjugate acid of ammonia. This means that ammonium ions in water will donate protons to water, regenerating the conjugate base (ammonia). Protonated water contains a molecular formula of , which are called hydronium ions.

Remember that hydronium ions are always produced during acid dissociation. The presence of more hydronium ions indicates that the solution is acidic and has a low pH.

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