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1 Chapter 8 Acids and Bases 8.4 The pH Scale Copyright © 2009 by Pearson Education, Inc.

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2 pH Scale The pH of a solution is used to indicate the acidity of a solution. has values that usually range from 0 to 14. is acidic when the values are less than 7. is neutral with a pH of 7. is basic when the values are greater than 7.

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3 pH of Everyday Substances

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4 Identify each solution as 1) acidic 2) basic3) neutral A. ___ HCl with a pH = 1.5 B. ___ pancreatic fluid [H 3 O + ] = 1 x 10 −8 M C. ___ Sprite ® soft drink pH = 3.0 D. ___ pH = 7.0 E. ___ [OH − ] = 3 x 10 −10 M F. ___ [H 3 O + ] = 5 x 10 −12 Learning Check

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5 A. 1 HCl with a pH = 1.5 B. 2 pancreatic fluid [H 3 O + ] = 1 x 10 −8 M C. 1 Sprite ® soft drink pH = 3.0 D. 3 pH = 7.0 E. 1 [OH − ] = 3 x 10 −10 M F. 2 [H 3 O + ] = 5 x 10 −12 Solution

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6 Testing the pH of Solutions The pH of solutions can be determined using a) pH meter. b) pH paper. c) indicators that have specific colors at different pH values.

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7 pH is the negative log of the hydronium ion concentration. pH = - log [H 3 O + ] Example: For a solution with [H 3 O + ] = 1 x 10 −4 pH =−log [1 x 10 −4 ] pH = - [-4.0] pH = 4.0 Note: The number of decimal places in the pH equals the significant figures in the coefficient of [H 3 O + ]. 4.0 1 SF in 1 x 10 -4 Calculating pH

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8 Guide to Calculating pH of an Aqueous Solution Copyright © 2009 by Pearson Education, Inc.

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9 Significant Figures in pH When expressing log values, the number of decimal places in the pH is equal to the number of significant figures in the coefficient of [H 3 O + ]. [H 3 O + ] = 1 x 10 -4 pH = 4.0 [H 3 O + ] = 8.0 x 10 -6 pH = 5.10 [H 3 O + ] = 2.4 x 10 -8 pH = 7.62

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10 Find the pH of a solution with a [H 3 O + ] of 1.0 x 10 −3 : STEP 1: Enter [H 3 O + ] Enter 1 x 10 -3 by pressing 1 (EE) 3 The EE key gives an exponent of 10 and change sign (+/- key or – key) STEP 2: Press log key and change sign - log (1 x 10 −3 ) = -[-3] STEP 3: Adjust figures after decimal point to equal the significant figures in the coefficient. 3 3.00 Two significant figures in 1.0 x 10 −3 Calculating pH

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11 Learning Check What is the pH of coffee if the [H 3 O + ] is 1 x 10 −5 M? 1) pH = 9.0 2) pH = 7.0 3) pH = 5.0

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12 Solution What is the pH of coffee if the [H 3 O + ] is 1 x 10 −5 M? 3) pH = 5.0 pH = -log [1 x 10 −5 ] = -(-5.0) = 5.0

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13 A. The [H 3 O + ] of tomato juice is 2 x 10 −4 M. What is the pH of the solution? 1) 4.02) 3.73) 10.3 B. The [OH − ] of a solution is 1.0 x 10 −3 M. What is the pH of the solution? 1) 3.002) 11.003) -11.00 Learning Check

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14 A. 2) 3.7 pH = - log [ 2 x 10 -4 ] = 3.7 2 (EE) 4 (+/-) log (+/-) B. 2) 11.00 Use the K w to obtain [H 3 O + ] = 1.0 x 10 − 11 pH = - log [1.0 x 10 − 11 ] 1.0 (EE) 11 (+/-) log (+/-) Solution

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15 [H 3 O + ], [OH − ], and pH Values Copyright © 2009 by Pearson Education, Inc.

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16 Calculating [H 3 O + ] from pH The [H 3 O + ] can be expressed by using the pH as the negative power of 10. [H 3 O + ] = 1 x 10 - pH For pH = 3.0, the [H 3 O + ] = 1 x 10 - 3 On a calculator 1. Enter the pH value 3.0 2. Change sign -3.0 3. Use the inverse log key (or 10 x ) to obtain the [H 3 0 + ]. = 1 x 10 - 3 M

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17 A. What is the [H 3 O + ] of a solution with a pH of 10.0? 1)1 x 10 −4 M 2)1 x 10 10 M 3) 1 x 10 −10 M B. What is the [OH − ] of a solution with a pH of 2.00? 1)1.0 x 10 − 2 M 2)1.0 x 10 −12 M 3)2.0 M Learning Check

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18 A. What is the [H 3 O + ] of a solution with a pH of 10.0? 3) 1 x 10 -10 M1 x 10 -pH B. What is the [OH − ] of a solution with a pH of 2.00? 2) 1.0 x 10 −12 M [H 3 O + ] = 1.0 x 10 −2 M 1 x 10 -pH [OH − ] = 1.0 x 10 −14 = 1.0 x 10 −12 M 1.0 x 10 −2 Solution

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