There space several definitions of what constitutes one acid:

The Arrhenius definition: through the 1884 definition 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 can accept a pair of electron and kind a name: coordinates covalent bond.

The Brønsted-Lowry definition: Brønsted said that every acid-base reactions involve the carry of one H+ ion, or proton. Water reacts with itself, for example, by moving an H+ ion from one molecule to an additional to kind an H3O+ ion and also an OH- ion. According to this theory, an mountain is a "proton donor" and also a base is a "proton acceptor."

For now we will stick v the Brønsted-Lowry definition. And we will currently tie the ide of acids and also bases right into equilibrium:

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

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

This reaction go fit the Brønsted-Lowry meaning since one water molecule is acting together an mountain "donating" a proton and the various other is acting together a base "accepting" the proton:


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


But remember one of our rules regarding the composing of equilibrium expressions states that we do not encompass solvents (liquids) together their concentration continues to be pretty consistent during a reaction so the water portion of the equation is basically equal to 1 and also thus "disappears".

The result equation is provided a unique designation KW called the ionization constant of water:


The worth of this continuous at 25oC is 1.0 x 10-14. As you can see indigenous the very tiny value the KW, water is not very dissociated in ~ room temperature and also our presumption that the concentration that water being consistent is a good one.

For pure water, the values of <H3O+> and also <OH-> space equal and thus your values space both 1.0 x 10-7. If over there is an equal amount that acid and also base existing in any kind of solution, the systems is referred to as "neutral" and the pH that the equipment is 7.

If the mountain concentration is greater than the basic concentration, the equipment is acidic and the pH will be much less than 7. If the basic concentration is higher than the acid concentration, the equipment is simple and the pH will be better than 7.

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The pH Scale:

The pH that a systems is characterized as the negative log10

(NOTE: H+ and also H3O+ room interchangeable means to define the visibility of protons in solution)

The pOH that a solution is characterized as the negative log10


= 10-pH and = 10-pOH

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

The amount of the pH and pOH must always equal 14. This is since the –log KW = -log(1 x 10-14) = 14.