Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Chapter 8 Acids and Bases 8.4 The pH Scale 1

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Rain Is Naturally Acidic Natural rain is slightly acidic with a pH of 5.6. Two reactions that take place in the atmosphere account for this: CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) H 2 CO 3 (aq) + H 2 O(l) H 3 O + (aq) + HCO 3  (aq) 2

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. The 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 3

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. pH of Everyday Substances 4 On the pH scale, values below 7.0 are acidic, a value of 7.0 is neutral, and values above 7.0 are basic.

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check Identify each solution as acidic, basic, or neutral. A. ___ HCl with a pH = 1.5 B. ___ pancreatic fluid, [H 3 O + ] = 1  10  8 M C. ___ Sprite soft drink, pH = 3.0 D. ___ pH = 7.0 E. ___ [OH  ] = 3  10  10 M F. ___ [H 3 O + ] = 5  10  12 5

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution Identify each solution as acidic, basic, or neutral. A. ___ HCl with a pH = 1.5 acidic B. ___ pancreatic fluid, [H 3 O + ] = 1  10  8 Mbasic C. ___ Sprite soft drink, pH = 3.0acidic D. ___ pH = 7.0neutral E. ___ [OH  ] = 3  10  10 Mbasic F. ___ [H 3 O + ] = 5  10  12 basic 6

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Acid Rain Forms from Fossil Fuels In some parts of the U.S. and Europe, the burning of fossil fuels and coal releases sulfur impurities that form SO 2 and SO 3 gases. These gases react with moisture in the air to form acid rain, lowering the pH of soil, lakes, and streams. 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) SO 3 (g) + H 2 O(l) H 2 SO 4 (aq) 7

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Acid Rain Forms from Fossil Fuels When the pH of lakes and streams falls below 4.5–5, most fish and plant life die. Trees and forests are damaged as the acid rain breaks down the waxy coating on leaves, interfering with photosynthesis. 8 Acid rain has severely damaged forests in Eastern Europe.

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Calculating pH of Solutions 9 pH is the negative log of the hydronium ion concentration. pH =  log [H 3 O + ] Example: For a solution with [H 3 O + ] = 1  10  4 pH =  log [1  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 + ] SF: 1  10  4

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Significant Figures in pH Calculations 10 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  10  4 pH = 4.0 [H 3 O + ] = 8.0  10  6 pH = 5.10 [H 3 O + ] = 2.4  10  8 pH = 7.62

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Guide to Calculating pH of Solutions 11

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Calculating pH Find the pH of a solution with a [H 3 O + ] of 1.0  10  3 : Step 1Enter the [H 3 O + ]. Enter 1  10  3 by pressing 1 (EE) 3. The EE key gives an exponent of 10. Enter change sign (+/  key or – key). Step 2Press the log key and change the sign.  log (1  10  3 ) =  [  3] 12

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Calculating pH Find the pH of a solution with a [H 3 O + ] of 1.0  10  3 : Step 3 Adjust the number of significant figures on the right of the decimal point to equal the SF in the coefficient. The answer is 3 with 2 decimal places: (Two significant figures in 1.0  10  3 ) 13

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check What is the pH of coffee if the [H 3 O + ] is 1  10  5 M? A. pH = 9.0 B. pH = 7.0 C. pH =

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution What is the pH of coffee if the [H 3 O + ] is 1  10  5 M? Step 1 Enter the [H 3 O + ]. Enter 1  10  5 by pressing 1 (EE) 5. The EE key gives an exponent of 10. Enter change sign (+/  key or – key). Step 2Press the log key and change the sign.  log (1  10  5 ) =  [  5] 15

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution What is the pH of coffee if the [H 3 O + ] is 1  10  5 M? Step 3 Adjust the number of significant figures on the right of the decimal point to equal the SF in the coefficient. The pH is 5 with 1 decimal place: 5.0, answer C. (One significant figure in 1  10  5 ) 16

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check The [H 3 O + ] of tomato juice is 2  10  4 M. What is the pH of the solution? A. 4.0B. 3.7C

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution 1.The [H 3 O + ] of tomato juice is 2  10  4 M. Step 1 Enter the [H 3 O + ]. Enter 2  10  4 by pressing 2 (EE) 4. The EE key gives an exponent of 10. Enter change sign (+/  key or – key). Step 2 Press the log key and change the sign.  log (2  10  4 ) =  [  3.7] Step 3 Adjust the number of significant figures on the right of the decimal point to equal the SF in the coefficient. pH =  log [ 2  10  4 ] = 3.7 The answer is B. 18

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. [H 3 O + ], [OH  ], and pH Values 19

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. 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  10  pH For pH = 3.0, the [H 3 O + ] = 1  10  3 On a calculator: 1. Enter the pH value Change sign  Use the inverse log key (or 10 x ) to obtain the [H 3 O + ]. = 1  10  3 M 20

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check 1. What is the [H 3 O + ] of a solution with a pH of 10.0? A.1  10  4 M B.1  M C.1  10  10 M 2. What is the [OH  ] of a solution with a pH of 2.00? A.1.0  10  2 M B.1.0  10  12 M C.2.0 M 21

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution 1. What is the [H 3 O + ] of a solution with a pH of 10.0? Using equation: 1  10  pH = 1  10  10 M The answer is C, 1  10  10 M. 2. What is the [OH  ] of a solution with a pH of 2.00? [H 3 O + ] = 1.0  10  2 M 1  10  pH [OH  ] = 1.0  10  14 = 1.0  10  12 M 1.0  10  2 The answer is B, 1.0  10  12 M. 22