ConjugateAcid-BasePairs(2)

　　pH = -log [H+] or -log [H3O+]

　　A solution like the equilibrium expression for water, which is neutral at standard temperature, would have a pH of

　　pH = -log [110-7] = -(-7.00) = 7.00

　　So as you can see, neutral solutions have a pH of 7. If the solution contains more hydronium ions than this neutral solution ([H+] > 110-7), the pH will be less than 7.00, and the solution will be acidic; if the solution contains more hydroxide ions than this neutral solution ([OH-] > 110-7), the pH will be 美国GREater than 7.00, and the solution will be basic.　　Similarly, the pOH of a solution is calculated as the negative logarithm in base 10 of the hydroxide ion concentration:

　　pOH = -log [OH-]

　　and pH and pOH are related to each other by the equation

　　pH + pOH = 14

　　Since you won’t be allowed to have a calculator for the SAT II Chemistry test, you can use the following equation if you need to calculate the hydronium ion concentration of a solution:

　　[H3O+] = 10-pH

　　Now try a problem: What is the pH of a solution at 25oC in which [OH-] = 1.010-5 M?　　Explanation　　The fact that this solution is at 25oC tells us that we should use the Kw relationships. If the [OH-] = 1.010-5 M, then pOH = 5. You know that 1.010-5 is the same as plain old 10-5. The log of 10-5 is -5 (simply use the exponent when a number, any number, is written as 10power, so the “negative” of the log is equal to -(-5), or simply 5. Now, if the pOH is 5, then the pH is 9 since pH + pOH = 14.

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