Titration of a weak acid with a strong base.

Titration of a weak acid with a strong base.

The definition of an acid or a base depends on the system you are using. In the Arrhenius definition of acids and bases, an acid is a substance that releases hydrogen ions when dissolved in water, and a base is a substance that releases hydroxide ions. Although in chemistry today, the Brønsted-Lowry definition has largely superseded it, under the Arrhenius definition it is possible to predict the result of the neutralization of an acid with a base, which is a salt and water.

If a chemist doesn't specify which acid-base system they are using, it can be assumed they are working within the Brønsted-Lowry system. A Brønsted acid is any substance that can donate a hydrogen ion. A Brønsted base is defined as any substance that can accept a hydrogen ion. In other words, a Brønsted acid is a proton donor, and a Brønsted base is a proton receiver. The Brønsted-Lowry system implies the acid-base reaction to occur within a system of conjugate acid-base pairs governed by an equilibrium. Acids increase the concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions. The acidity or basicity of a solution can therefore can be measured by its hydrogen ion concentration (or pH, the negative logarithm of concentration).

The Lewis definition of acids (as electron pair receivers) and bases (as electron pair donors) is actually broader than either the Arrhenius or Brønsted-Lowry. The Lewis system provides a productive framework for viewing many chemical events from nucleophilic approach to the formation of coordination complexes, although the Brønsted-Lowry system is the traditional framework for discussion and problem solving involving acid-base equilibria.

As with Solutions, the previous chapter, every MCAT will have a number of questions deriving from the material in Acids & Bases. Although big, multi-variable quantitative acid-base equilibrium questions are a standard of Chem 101, you are not likely to run into such questions on the MCAT. However, the MCAT definitely will call on you to demonstrate that you understand the basic concepts of acid-base equilibrium. Buffers & indicators are MCAT favorites, as well as titration curves.

WikiPremed Resources

Acids & Bases Practice Items
Problem set for Acids & Bases in PDF format

Answer Key
Answers and explanations

Acids and Bases Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Acids & Bases
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Clearly distinguish the systems for defining acids and bases including Arrhenius, Brønsted-Lowry, and Lewis.

Understand the meaning of conjugate acids and bases in the Brønsted-Lowry system.

Be able to place the Brønsted-Lowry system in the aqueous context in which the equilibrium of a conjugate acid and base is coupled with the autoprotolysis of water.

Place acidity and basicity in the context of organic reaction chemistry. Become familiar with the reactions in which it is an important factor.

Become fluent in working with pH to describe the position of acid-base equilibrium in an aqueous solution.

Be prepared to predict the effect on acidity or basicity of an electron withdrawing or electron donating substituent placed near to an acidic or basic organic functional group.

Understand how to employ the the acid constant ka to determine the strength of an acid or its conjugate base, and become fluent in the basic techniques for solving acid-base equilibrium problems.

Develop a concrete, intuitive understanding of how a buffered solution works to modulate pH.

Know how to work with the Henderson-Hasselbalch equation backwards and forwards.

Be able to explain the behavior of an indicator.

Be able to interpret the titration curve of a weak acid with a strong base and the titration curve of a weak base with a strong acid. Be able to pick out the half-equivalence point and the equivalence point.

Extend your understanding to other titration curves, such as the titration of a polyprotic acid or an amino acid.

Suggested Assignments

Study the terminology for acids & bases using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the practice items for acids & bases. Here is the answer key for the practice items.

Read pp. 207-216, 218-221 and 223-234 in ExamKrackers Chemistry. Perform practice items 145-152 on pg. 217, 153-160 on pg. 222, and 161-168 on pp. 235-236.

Review the web resources for acids & bases.

Conceptual Vocabulary for Acids & Bases

Acids & Bases

Each list begins with basic conceptual vocabulary you need to know for MCAT questions and proceeds to advanced terms that might appear in context in MCAT passages. The terms are links to Wikipedia articles.
An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a pH less than 7.0.
A base is most commonly thought of as a substance that can accept protons.
pH is a measure of the acidity or alkalinity of a solution.
Strong acid
A strong acid is an acid that dissociates completely in an aqueous solution.
Acetic acid
Acetic acid, also known as ethanoic acid, is an organic chemical compound best recognized for giving vinegar its sour taste and pungent smell.
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of a carboxyl group.
Sodium hydroxide
Sodium hydroxide, also known as lye, caustic soda and sodium hydrate, is a caustic metallic base.
Hydronium is the common name for the cation derived from protonation of water. It is the simplest type of an oxonium ion.
Acid-base reaction
An acid-base reaction is a chemical reaction that occurs between a proton donor and a proton receiver.
Lewis base
A Lewis base is any molecule or ion that can form a new coordinate covalent bond, by donating a pair of electrons.
Organic base
Amines and nitrogen-containing heterocyclic compounds are organic bases.
Amines are organic compounds and a type of functional group that contain nitrogen as the key atom. Structurally amines resemble ammonia, wherein one or more hydrogen atoms are replaced by alkyl and aryl groups.
Hydrochloric acid
Hydrochloric acid is the aqueous solution of hydrogen chloride gas.
Lewis acid
A Lewis acid can accept a pair of electrons and form a coordinate covalent bond.
Weak base
A weak base is a chemical base that does not ionize fully in an aqueous solution
Weak acid
A weak acid is an acid that does not ionize in solution to a significant extent.
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom.
An alkali is a basic, ionic salt of a group I or group II element.
Sodium carbonate
Sodium carbonate is a sodium salt of carbonic acid.
Acid dissociation constant
An acid dissociation constant is an equilibrium constant for the dissociation of a weak acid.
Buffer solution
Buffer solutions are solutions that resist change in Hydronium ion and the hydroxide ion concentration (and consequently pH) upon addition of small amounts of acid or base, or upon dilution.
Acid-base titration
An acid-base titration is a volumetric method in chemistry that allows quantitative analysis of the concentration of an unknown acid or base solution, making use of the neutralization reaction that occurs between acids and bases.
Protonation is the addition of a hydrogen ion to an atom, molecule, or ion.
pH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined easily.
An acetate, or ethanoate, is a salt or ester of acetic acid.
Ammonia is a compound consisting of one nitrogen atom singly bound to three hydrogen atoms, normally encountered as a gas with a characteristic pungent odor.
Hydrogen chloride
Hydrogen chloride has the formula HCl.
Sulfuric acid
Sulfuric acid is a strong mineral acid once known as oil of vitriol. Each molecule contains a sulfur atom, two hydrogen atoms and four oxygen atoms.
Equivalence point
The equivalence point or stoichiometric point occurs during a chemical titration when the amount of titrant added is equivalent, or equal, to the amount of analyte present in the sample.
Mineral acid
A mineral acid is an acid derived from inorganic substances by chemical reaction as opposed to organic acids.
Neutralization is a chemical reaction, also called a water forming reaction, in which an acid and a base react and produce a salt and water.
Phenol, also known under an older name of carbolic acid, possesses a structure consisting of a hydroxyl group bonded to a phenyl ring.
Benzoic acid
Benzoic acid is a colorless crystalline solid and the simplest aromatic carboxylic acid.
An oxoacid is an acid which contains oxygen.
Nitric acid
Nitric acid, also known as aqua fortis and spirit of nitre, is an aqueous solution of hydrogen nitrate.
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid, is a mineral acid having a molecular structure with one phosphorus atom, four oxygen atoms, and three hydrogen atoms.
A bicarbonate is an intermediate form in the deprotonation of carbonic acid.
Nitrogenous base
Nitrogenous bases are organic compounds that owe their basic properties to the lone pair of electrons of a nitrogen atom.
Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond.
A superbase is an extremely strong base.
A superacid is an acid with an acidity greater than that of 100% sulfuric acid.
Proton affinity
The proton affinity of an anion or of a neutral atom or molecule is a measure of its gas-phase basicity.
Hydron is the general name for the positive hydrogen cation, used to reflect ions formed from the naturally abundant hydrogen, as opposed to proton, which only refers to the most common isotope of hydrogen.
A burette is a vertical cylindrical piece of laboratory glassware with a volumetric graduation on its full length and a precision tap, or stopcock, on the bottom.
Formic acid
Formic acid (systematically called methanoic acid) is the simplest carboxylic acid
Malonic acid
Malonic acid is a dicarboxylic acid with two carboxyl groups bound to a central carbon.
Basic aromatic ring
Basic aromatic rings are aromatic rings in which the lone pair of electrons of a ring-nitrogen atom is not part of the aromatic system and extends in the plane of the ring.
Methylamine is a derivative of ammonia, wherein one H atom is replaced by a methyl group
Harpoon base
A harpoon base is an organic base that is a very strong base but at the same time a poor nucleophile.
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