2. The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. Pure water is neutral. But when chemicals are mixed with water, the mixture can become either acidic or basic. Examples of acidic substances are vinegar and lemon juice. Lye, milk of magnesia, and ammonia are examples of basic substances.

4. Effect of Acids and Bases on Water Equilibrium: If an acid (H + ) is added to the water, the equilibrium shifts to the left and the OH - ion concentration decreases. Water Equilibrium: H 2 O  H + + OH - If base ( OH - ) is added to water, the equilibrium shifts to left and the H+ concentration decreases. Water Equilibrium Principle: The multiplication product (addition of exponents) of H + and OH - ion concentration must always be equal to 10 -14. BOTH H + and OH - ions are ALWAYS PRESENT in any solution. A solution is acidic if the H + are in excess. A solution is basic, if the OH - ions are in excess.

5. Definition of pH, pOH, and pK w : The concentrations of hydrogen ions and indirectly hydroxide ions are given by a pH number. pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is:pH = - log [H + ] similarly, pOH = - log [OH - ]and p K w = - log [K w ]. Since K w = 10 -14 pK w = 14pK = pH + pOH therefore pH + pOH = 14

6. Logarithms of numbers that are multiples of ten are merely the exponents of the number including the sign. See the table on the left for a review. The method to find logs of numbers that are not multiples of ten are found by using a calculator. Number Representation and Logarithms Number Exponent Notation Log of the Number 1000 103 3 100 102 2 10 101 1 1 100 0 0.1 10-1 -1 0.01 10-2 -2 0.001 10-3 -3 0.0001 10-4 -4

7. Example: If an acid has an H+ concentration of 0.0001 M, find the pH. Solution: First convert the number to exponential notation, find the log, then solve the pH equation H+ = 0.0001M = 10 -4 ; log of 10 -4 = - 4; pH = - log [ H+] = - log (10 -4 ) = - (-4) = +4 = pH The purpose of the negative sign in the log definition is to give a positive pH value.

9. pH Scale Principle: H + ion concentration and pH relate inversely. OH - ion concentration and pH relate directly. The following statements may be made about the pH scale numbers. Complete some of them. a.Increasing pH means the H + ions are decreasing. b. Decreasing pH means H + ions are increasing. c.Increasing pH means OH - ions are _______ d.Decreasing pH means OH - ions are ______

10. What is the pH of a solution whose [H + ] = 1.65 x 10 -4 M? What is the pOH of the solution? What is the [OH] of a solution whose pH = 4.57?