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**Acids and Bases pH Calculations**

pH scale

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pH Scale Created to express acidity as a more simple number than molarity. pH stands for “power” of the hydrogen ion concentration. pH = - log [H+] If [H+] = 1 x then pH = 7 (neutral) [H+] = 1 x then pH = 3 (acid) [H+] = 1 x then pH = 10 (base) It is based on a logarithmic scale which has a base power of 10. A pH of 1 differs from a pH of 2 by a factor of 10.

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How to calculate pH…. What is the pH of a solution with a [H+] = 4.2 x 10-10M? pH= -log [H+] = -log (4.2 x 10-10) = 9.38 Enter into calculator like this or, Enter 4.2 x in calculator and hit log button, then remove (-) sign.

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**To find the [H+] from pH [H+] = 10-pH**

Determine the [H+] of a solution with a pH = 6.35. [H+] = 10-pH [H+] = [H+] = 4.47 x 10-7 M Find the “log” button on your calculator Above this button it should say “10x” You can use the 2nd key and this button to “un-log” the pH. But, remember to put the (-) sign on the pH first!!

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**pOH We can also express the [OH-] as pOH pOH = -log [OH-]**

pOH stands for the “power of the hydroxide ion.” pOH<7 is basic pOH=7 is neutral pOH>7 acidic

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**pH and pOH together pH + pOH =14**

What is the pH of a solution which has a pOH of 11.2 ? Is this solution acidic or basic? pH = 14 pH = 2.8 So, the solution is acidic.

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**What is the pH of a solution with a [OH-] = 4.0 x 10–11M?**

Step 1: Find pOH pOH = -log[OH-] = -log (4.0 x 10-11) = 10.40 Step 2: Find pH using pH + pOH = 14 pH = 14 pH = 3.60

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Ch. 18: Acids & Bases Sec. 18.3: What is pH?. Objectives n Explain the meaning of pH and pOH. n Relate pH and pOH to the ion product constant for water.

Ch. 18: Acids & Bases Sec. 18.3: What is pH?. Objectives n Explain the meaning of pH and pOH. n Relate pH and pOH to the ion product constant for water.

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