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.

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.

 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.

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.

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.

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.

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.

NH₄⁺ is the conjugate acid of ammonia (NH₃), a weak base. As a result, it will donate protons to the solution and make the solution acidic.

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.

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)

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.