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Self-ionization of Water
Water can self ionize, which means that if conditions are right, two molecules of water can produce a hydronium ion and a hydroxide ion, as shown below:
Water can self ionize, which means that if conditions are right, two molecules of water can produce a hydronium ion and a hydroxide ion, as shown below:
2 H2O (l) <==> H3O+ (aq) + OH- (aq)
Essentially what happens is that water will dissolve itself. When pure water dissolves, it creates equal amounts of acid (H3O+) and base (OH-). Every water solution has these two ions in it. An acid solution will have more H3O+ than OH-, and vice versa for base. Regardless, of whether the solution is an acid or a base, the following equality is always true:
1.0 x 10-14 = [H3O+] [OH-]
This equality means that for every water solution (whether it is acidic or basic), the molarity of the hydronium times the molarity of the hydroxide will always equal 1.0 x 10-14.
Example 1: What is the concentration of [H3O+] in an orange juice solution when the [OH-] is equal to 1.5 x 10-10 M?
Example 1: What is the concentration of [H3O+] in an orange juice solution when the [OH-] is equal to 1.5 x 10-10 M?
Answer: 1.0 x 10-14 = [H3O+] [1.5 x 10-10]
[H3O+] = 6.7 x 10-5 M
pH
The pH scale, designed by Sorensen, was a proposal that expresses acidity and basicity in a more compact form. Using the molarities of H3O+ and OH- to determine whether a solution is acidic or basic is confusing. The pH scale is designed to make the numbers easier to work with. Since the molar concentration of hydronium is different in different substances, we use a scale to show this concentration.
The pH scale appears at right: To determine the pH from the hydronium concentration, we use the following formula: pH = -log [H3O+]
On the pH scale, acids range from 0 to just below 7. 7 is neutral and from just above 7to14 is the bases. The farther you get from 7, the more acidic or basic the solution is. Example 2: What is the pH of the orange juice solution in example #1? Answer: Since the H3O+ is equal to 6.7 x 10-5 M, then
pH = -log [6.7 x 10-5]
pH = 4.18 A pH of 4.18 is in the acid range of the pH scale. pOH |
The pOH scale is similar to the pH scale, except pOH is a scale to show the concentration of OH- ions in solution. The pOH scale is based off of the molarity of hydroxide ions in solution:
pOH = -log [OH-]
Example 3: What is the pOH of the orange juice solution in example #1?
Answer: Since the OH- is equal to 1.5 x 10-10 M, then
Answer: Since the OH- is equal to 1.5 x 10-10 M, then
pOH = -log [1.5 x 10-10]
pOH = 9.82
pOH = 9.82
This substance is still an acid. A pOH of 9.82 would be acidic. On the pOH scale, acids will range from above 7 to 14. Bases range from 0 to just below 7.