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 #1 : Defining/Classifying Acids And Bases

None of the questions in this set require the use of a calculator. Math problems are intended to mimic the level of math intensity that you will see on the MCAT exam.

A Bronsted-Lowry acid will be able to __________.

Possible Answers:

accept a proton

donate a pair of electrons

donate a proton

donate an electron

donate a hydrogen atom

Correct answer:

donate a proton

Explanation:

The Bronsted-Lowry definition of an acid is one of three acid definitions that can be used on the MCAT. The three definitions are as follows:

Arrhenius acid: increases H+ concentration in water (releases an H+)

Bronsted-Lowry acid: donates a proton to a Bronsted base (Bronsted bases accept protons)

Lewis acid: accepts a pair of electrons from a Lewis base (Lewis bases donate electrons)

An easy way to remember the difference between Bronsted-Lowry and Lewis is that the "e" in Lewis comes before the "e" in Bronsted; therefore the Lewis definition has to do with electrons.

Example Question #2 : Defining/Classifying Acids And Bases

Which of the following describes a Brønsted-Lowry acid?

Possible Answers:

A substance that produces hydroxide ions in an aqueous solution

A substance that donates a proton

A substance that donates an electron pair

A substance that accepts an electron pair

Correct answer:

A substance that donates a proton

Explanation:

There are three definitions for an acid that are commonly used: an Arrhenius acid, a Brønsted-Lowry acid, and a Lewis acid. Arrhenius acids are any substances that create hydrogen ions in solution, and Lewis acids are any substances that accept an electron pair. A Brønsted-Lowry acid is defined as any substance that donates a proton.

Example Question #3 : Defining/Classifying Acids And Bases

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.

Reducing agents, such as , are used to donate hydrogen ions to chemical species.  will dissociate in solution to form hydrogen ions, which then reduce the target compound.

Which of the following must be true of  when it is used as a reducing agent?

Possible Answers:

It is not an acid, as it donates hydride ions and not protons

It must be an acid, as it donates protons and not hydride ions

 is not an acid, but the reduced compound must be a base

It is not an acid, as it donates protons and not hydride ions

It must be an acid, as it donates hydride ions and not protons

Correct answer:

It is not an acid, as it donates hydride ions and not protons

Explanation:

 is not an acid when it is used as a reducing agent. This can confuse students because,  is in fact donating a hydrogen ion to a compound that is being reduced; however, we are reducing something, and thus we are adding electrons. The only way that  could be an effective reducing agent is if it carried electrons on its donated hydrogen.

As a result, we are donating hydride () ions, and not protons. Acids are exclusively those species that donate protons in solution, not hydrides.

Example Question #1 : Defining/Classifying Acids And Bases

Which of the following is not a Lewis acid?

Possible Answers:

Correct answer:

Explanation:

The definition of a Lewis acid is an electron acceptor. Try drawing the Lewis dot structures for these compounds, and you will find that NH4+, Na+, and Al3+ are missing electrons. They each have an empty orbital, allowing them to accept electrons.

BF3 is not an ion; however, we know that boron only has 3 valence electrons, which means that even when they are all bound to fluorine the molecule does not satisfy the octet rule. BF3 only has 6 valence electrons around boron, and can accept another electron pair to get to the octet state.

CaO is a Lewis base. In the Lewis dot structure, we can see that the oxygen molecule has two lone pairs that it can donate to other molecules, making it an electron donor.

Example Question #5 : Acids And Bases

Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.

CO2 + H2O  H2CO3

 H2CO3  HCO3- + H+

HCO3- is the __________ of H2CO3.

Possible Answers:

base

acid

conjugate acid

conjugate base

Correct answer:

conjugate base

Explanation:

First, we need to determine what type of reaction HCO3- is participating in. We can see that a H+ is being produced in the forward reaction, and consumed in the reserve reaction; thus, we are looking at an acid-base reaction. H2CO3 has lost a H+, making it a Arrhenius acid. HCO3- is the conjugate base of the acid H2CO3.

Example Question #2 : Defining/Classifying Acids And Bases

Which of the following acids has the weakest conjugate base?

Possible Answers:

HClO2

HF

H2CO3

HNO3

Correct answer:

HNO3

Explanation:

The stronger the acid, the weaker its conjugate base. This is because a strong acid will dissociate almost entirely, meaning that its conjugate base will not accept protons frequently. The only strong acid as an option is nitric acid (HNO3). As a result, we conclude that nitric acid has the weakest conjugate base.

Example Question #6 : Defining/Classifying Acids And Bases

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

Possible Answers:

NaCl

LiBr

NH4NO3

NaF

Correct answer:

NH4NO3

Explanation:

Certain salts are able to affect the pH of a solution, because the dissociated ions can act as acids or bases. The conjugate bases of strong acids will not affect the pH, nor will the conjugate acids of strong bases.

NH4is the conjugate acid of ammonia (NH3), a weak base. As a result, it will donate protons to the solution and make the solution acidic.

Example Question #7 : Defining/Classifying Acids And Bases

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.

Instead of a monoprotic acid like , a scientist is studying a diprotic acid, like . Which of the following is true regarding ?

I. It only lowers the pH in solution when both protons are released

II. It releases a much greater fraction of its first proton than its second

III.  is its conjugate acid

Possible Answers:

I and III

III only

I, II, and III

I only

II only

Correct answer:

II only

Explanation:

A diprotic acid will ionize fewer of its second protons than its first; that is, a smaller fraction of the  ions will react to  than molecules will react to .

The presence of protons in solution from the original ionization of  in the first reaction drives the equilibrium toward  (reactants) and away from  (products) in the second reaction.

Example Question #9 : Acids And Bases

Which of the following will dissolve completely in water?

Possible Answers:

HCN

NH3

HCOOH

HClO4

C6H5COOH

Correct answer:

HClO4

Explanation:

The question is asking which of the following is a strong acid. Of the following, the only strong acid provided is perchloric acid, HClO4. NH3 is a weak base; HCOOH, C6H5COOH, and HCN are all weak acids. A strong acid is one that dissociates completely in water.

The strong acids that you must know for the MCAT are:

Hydroiodic acid (HI)

Hydrobromic acid (HBr)

Hydrochloric acid (HCl)

Perchloric acid (HClO4)

Sulfuric acid (H2SO4)

and Nitric acid (HNO3)

Example Question #8 : Defining/Classifying Acids And Bases

Which of the following is the strongest acid?

Possible Answers:

HF

HI

HCl

HBr

Correct answer:

HI

Explanation:

Hydroiodic acid (HI) is the strongest acid listed. Charge density decreases as the atomic size gets larger, thus stabilizing the charge when a hydrogen is given off. Hydroflouric acid (HF) is a relatively weak acid because electronegative elements hold on to their valence electrons more tightly, and are thus less likely to dissociate.

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