There are numerous interpretations of what constitutes an acid:

The Arrhenius definition: By the 1884 definition of Svante Arrhenius (Sweden), an acid is a material that can release a proton or hydrogen ion (H+).

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The Lewis definition: A Lewis acid is one that have the right to accept a pair of electrons and also develop a coordinate covalent bond.

The Brønsted-Lowry definition: Brønsted said that all acid-base reactions involve the move of an H+ ion, or proton. Water reacts through itself, for instance, by carrying an H+ ion from one molecule to another to form an H3O+ ion and an OH- ion. According to this theory, an acid is a "proton donor" and also a base is a "proton acceptor."

For currently we will certainly stick with the Brønsted-Lowry interpretation. And we will currently tie the concept of acids and bases into equilibrium:

Let"s start by looking at the Ionization of Water:

2H2O(l) H3O+(aq) + OH-(aq)

This reactivity does fit the Brønsted-Lowry meaning considering that one water molecule is acting as an acid "donating" a proton and the other is acting as a base "accepting" the proton:


If we now write this reaction right into an equilibrium expression:


But remember one of our rules concerning the writing of equilibrium expressions states that we carry out not incorporate solvents (liquids) as their concentration remains pretty consistent in the time of a reaction so the water percent of the equation is basically equal to 1 and also for this reason "disappears".

The resulting equation is offered a unique designation KW dubbed the ionization continuous of water:


The worth of this continuous at 25oC is 1.0 x 10-14. As you deserve to watch from the extremely tiny value of KW, water is not very dissociated at room temperature and also our assumption that the concentration of water being constant is a good one.

For pure water, the worths of <H3O+> and also <OH-> are equal and also hence their values are both 1.0 x 10-7. If there is an equal amount of acid and also base existing in any kind of solution, the solution is dubbed "neutral" and also the pH of the solution is 7.

If the acid concentration is better than the base concentration, the solution is acidic and the pH will certainly be much less than 7. If the base concentration is greater than the acid concentration, the solution is fundamental and the pH will certainly be greater than 7.

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The pH Scale:

The pH of a solution is defined as the negative log10

(NOTE: H+ and also H3O+ are interchangeable methods to define the presence of proloads in solution)

The pOH of a solution is characterized as the negative log10


= 10-pH and = 10-pOH

The pH of a neutral solution is therefore 7. (-log<1 x 10-7> = 7)

The sum of the pH and pOH need to always equal 14. This is bereason the –log KW = -log(1 x 10-14) = 14.