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.

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.

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.

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.

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 .

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.

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.

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)

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.