The Chemistry of Acids and Bases

Acids and Bases: Define pH/pOH At the conclusion of our time together, you should be able to: 1. Define pH and pOH 2. Explain the pH scale and what it means 3. List 3 different ways to test for pH

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The pH scale is a way of expressing the strength of acids and bases. This scale is defined by: pH = - log [H+] Under 7 = acid 7 = neutral Over 7 = base What is pH???

 p = count  H = Hydronium ion  (H 3 O + )  In 1 Liter of water, (55+ moles of water), how many moles of hydronium ions are there???  Understanding pH gives me this number!!!  If there was 1 mole of H 3 O + in 1 Liter of water, (55+ moles of water), this count would be 1 mole.

What is pH???  Remember: pH = - log [H+]  pH = -log[1]  Therefore, by definition, the pH of this solution is  = 0

What is the pH of water???  H 2 O + H 2 O ---> H 3 O + + OH -  <===  This equation indicates that most water molecules stay as molecules, very few will form hydronium ions  If we count the number of hydrogen ions in one liter of water (pH) we would get:  moles of H 3 O + /liter of water  or moles of H 3 O + /liter of water  What is the pH of water??  pH = 7

What is the pOH of water???  H 2 O + H 2 O ---> H 3 O + + OH -  <===  This equation indicates that most water molecules stay as molecules, very few will form hydroxide ions  If we count the number of hydroxide ions in one liter of water (pOH) we would get:  moles of OH - /liter of water  or moles of OH - /liter of water  What is the pOH of water??  pOH = 7

How High is the pH Scale??  Since 1 mole of OH- in one Liter of water would be a pOH = 0  And the pH and pOH of water is 7 or neutral,  Therefore, the pH of a strong base with a pOH of 0 would be?  pH = 14  Therefore, the pH scale goes from 0-14  Also, any pH can be converted to pOH by?  Subtracting from 14.

pH- The Amount Of H + Ions In A Solution. ACID extra H +1 ions (protons) BASE fewer H +1 ions. extra OH -1 ions

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The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 of the Molarity of the H + (or OH - ) ion. Under 7 = acid 7 = neutral Over 7 = base

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pH Testing  There are several ways to test pH  Blue litmus paper (red = acid)  Red litmus paper (blue = basic)  pH paper (multi-colored)  pH meter (7 is neutral, 7 base)  Universal indicator (multi-colored)  Indicators like phenolphthalein  Natural indicators like red cabbage, radishes

Paper Testing  Paper tests like litmus paper and pH paper  Put a stirring rod into the solution and stir.  Take the stirring rod out, and place a drop of the solution from the end of the stirring rod onto a piece of the paper  Read and record the color change. Note what the color indicates.  Use a small portion of the paper. You can use one piece of paper for several tests.

pH Paper Blue Litmus paper Red Litmus paper

pH Indicators  Indicators are dyes that can be added that will change color in the presence of an acid or base.  Some indicators only work in a specific range of pH  Once the drops are added, the sample is ruined  Some dyes are natural, like radish skin or red cabbage

pH Meter  Tests the voltage of the electrolyte  Converts the voltage to pH  Expensive, accurate  Must be calibrated with a buffer solution

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pH of Common Substances

Acids and Bases: pH/pOH Let’s see if you can: 1. Define pH and pOH 2. Explain the pH scale and what it means 3. List 3 different ways to test for pH

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The “p” in pH stands for? 1. Priority 2. Count 3. Number 4. Molarity 5. Not listed

The “H” in pH stands for? 1. House 2. Hydrogen ions 3. Acid 4. Concentration 5. Not listed

Red litmus paper will turn what color in an acid? 1. Blue 2. White 3. Red 4. Clear 5. Not listed

Which of the following would be the most accurate way to test for pH? 1. pH meter 2. Litmus paper 3. Phenylthalein 4. Cabbage juice 5. Universal indicator 6. Not listed

Several liquids are tested for pH. Which of these liquids would be the weakest acid? 1. Liquid a, pH = Liquid b, pH = Liquid c, pH = Liquid d, pH = Liquid e, pH = Liquid f, pH = 13.0