There space several definitions of what constitutes an acid:

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

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The Lewis definition: A Lewis mountain is one that deserve to accept a pair of electron and form a coordinate covalent bond.

The Brønsted-Lowry definition: Brønsted argued that every acid-base reactions involve the deliver of an H+ ion, or proton. Water reacts v itself, for example, by carrying an H+ ion native one molecule to one more to kind an H3O+ ion and an OH- ion. Follow to this theory, an mountain is a "proton donor" and also a basic is a "proton acceptor."

For now we will stick with the Brønsted-Lowry definition. And we will now tie the principle of acids and bases into equilibrium:

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

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

This reaction does fit the Brønsted-Lowry meaning since one water molecule is acting together an acid "donating" a proton and also the 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 about the creating of equilibrium expressions claims that we perform not include solvents (liquids) together their concentration continues to be pretty consistent during a reaction for this reason the water section of the equation is essentially equal come 1 and also thus "disappears".

The result equation is provided a special designation KW referred to as the ionization continuous of water:


The value of this consistent at 25oC is 1.0 x 10-14. As you deserve to see native the very small value the KW, water is not an extremely dissociated in ~ room temperature and our assumption that the concentration the water being constant is a good one.

For pure water, the worths of <H3O+> and also <OH-> space equal and also thus your values are both 1.0 x 10-7. If there is an equal amount that acid and base current in any type of solution, the systems is dubbed "neutral" and the pH the the equipment is 7.

If the mountain concentration is higher than the base concentration, the systems is acidic and also the pH will be much less than 7. If the base concentration is higher than the acid concentration, the solution is straightforward and the pH will certainly be better than 7.

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The pH Scale:

The pH of a systems is identified as the negative log10

(NOTE: H+ and H3O+ room interchangeable ways to define the presence of proton in solution)

The pOH that a equipment is defined as the negative log10


= 10-pH and also = 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 constantly equal 14. This is since the –log KW = -log(1 x 10-14) = 14.