Tright here are several meanings of what constitutes an acid:

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

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The Lewis definition: A Lewis acid is one that deserve to accept a pair of electrons and also create a coordinate covalent bond.

The Brønsted-Lowry definition: Brønsted argued that all acid-base reactions involve the transfer of an H+ ion, or proton. Water reacts with itself, for example, by transferring an H+ ion from one molecule to an additional to form an H3O+ ion and also an OH- ion. According to this concept, an acid is a "proton donor" and a base is a "proton acceptor."

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

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

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

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


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


But remember one of our rules regarding the composing of equilibrium expressions claims that we perform not include solvents (liquids) as their concentration continues to be pretty constant during a reactivity so the water percentage of the equation is fundamentally equal to 1 and also hence "disappears".

The resulting equation is provided a distinct desigcountry KW called the ionization continuous of water:


The value of this consistent at 25oC is 1.0 x 10-14. As you have the right to watch from the very small worth of KW, water is not very dissociated at room temperature and also our presumption that the concentration of water being continuous is a great one.

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

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

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The pH Scale:

The pH of a solution is characterized as the negative log10

(NOTE: H+ and H3O+ are interchangeable methods to explain the presence of protons 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 amount of the pH and pOH need to always equal 14. This is bereason the –log KW = -log(1 x 10-14) = 14.