Chapter 10 Acids and Bases

Chapter 10 Acids and Bases 10.4The pH Scale

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

pH of Everyday Substances

Learning Check Identify each solution as A) acidic, B) basic, orN) neutral ___ 1) HCl with a pH = 1.5 ___ 2) pancreatic fluid [H3O+] = 1 x 10−8 M ___ 3) Sprite soft drink, pH = 3.0 ___ 4) pH = 7.0 ___ 5) [OH−] = 3 x 10−10 M ___ 6) [H3O+ ] = 5 x 10−12 M

Solution Identify each solution as A) acidic, B) basic, orN) neutral A 1) HCl with a pH = 1.5 B 2) Pancreatic fluid [H3O+] = 1 x 10−8 M A 3) Sprite soft drink pH = 3.0 N 4) pH = 7.0 A 5) [OH-] = 3 x 10−10 M B 6) [H3O+] = 5 x 10−12

Testing the pH of Solutions The pH of solutions can be determined using • a pH meter • pH paper • indicators that have specific colors at different pH values

Calculating pH • Mathematically, pHis the negative log of the hydronium ion concentration pH = −log [H3O+] • For a solution with [H3O+] = 1 x 10−4, pH = −log [1 x 10−4 ] pH = [4.0] pH = 4.0

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 [H3O+]. coefficient decimal places [H3O+] = 1 x 10−4 pH = 4.0 [H3O+] = 8.0 x 10−6 pH = 5.10 [H3O+] = 2.4x 10−8 pH = 7.62

Guide to Calculating pH

Example of Calculating pH Find the pH of a solution with a [H3O+] of 1.0 x 10−3. STEP 1Enter the [H3O+] value: Enter 1 x 103 (press1 EE 3, then change sign) The EE key gives the exponent of 10. STEP 2Press log key and change the sign: log (1 x 10−3) = [3] STEP 3 Make the number of digits after the decimal point (2) equal to the number of significant figures in the coefficient (2): [H3O+] = 1.0x 10−3 pH is 3.00

Learning Check What is the pH of coffee if the [H3O+] is 1 x 10−5 M? 1) pH = 9.0 2) pH = 7.0 3) pH = 5.0

Solution What is the pH of coffee if the [H3O+] is 1 x 10−5 M? STEP 1Enter the [H3O+] value: Enter 1 x 105 (press1 EE 5, then change sign ) STEP 2Press log key and change the sign: log (1 x 10−5) = [5] STEP 3 Make the number of digits after the decimal point (1) equal to the number of significant figures in the coefficient (1): [H3O+]= 1x 10−5, pH is 5.0(3)

Learning Check The [H3O+] of tomato juice is 2 x 10−4 M. What is the pH of the solution? 1) 4.0 2) 3.7 3) 10.3

Solution The [H3O+] of tomato juice is 2 x 10−4 M. What is the pH of the solution? STEP 1Enter the [H3O+] value: Enter 2 x 104 (press2 EE 4, then change sign ) STEP 2Press log key and change the sign: log (2 x 10−4) = [3.7] STEP 3 Make the number of digits after the decimal point (1) equal to the number of significant figures in the coefficient (1): [H3O+] = 2 x 10−4, pH is = 3.7(2)

Learning Check The [OH−] of a solution is 1.0 x 10−3 M. What is the pH? 1) 3.00 2) 11.00 3) –11.00

Solution The [OH−] of a solution is 1.0 x 10−3 M. What is the pH? STEP 1Enter the [H3O+] value: Use the Kw to obtain [H3O+] = 1.0 x 10−11 M Enter 1.0 x 10−11 (press1 EE 11, then change sign) STEP 2Press log key and change the sign: log (1.0 x 10−11) = [11] STEP 3 Make the number of digits after the decimal point (2) equal to the number of significant figures in the coefficient (2): [H3O+] =1.0x 10−11, pH is = 11.00(2)

[H3O+], [OH-], and pH Values

Example 1: Calculating [H3O+] from pH Calculate the [H3O+] for a pH value of 8.0. [H3O+] = 1 x 10−pH For pH = 8.0, the [H3O+] = 1 x 10−8 STEP 1Enter the pH value, change sign: –8.0 STEP 2Convert pH to concentration: Use 2ndfunction key and then10x key or inverse key and then log key 1 −08 STEP 3 Adjust the significant figures in the coefficient (1 digit following decimal point = 1 digit in the coefficient): [H3O+]=1 x 10−8 M

Example 2: Calculating [H3O+] from pH Calculate the [H3O+] for a pH of 3.80. STEP 1Enter the pH value, change sign: –3.80 STEP 2Convert pH to concentration: Use 2ndfunction key and then10x key or inverse key and then log key 1.584893 −06 STEP 3 Adjust the significant figures in the coefficient (2 digit following decimal point = 2 digit in the coefficient): [H3O+]=1.6 x 10−6 M

Learning Check What is the [H3O+] of a solution with a pH of 10.0? 1) 1 x 10−4 M 2) 1 x 1010 M 3) 1 x 10−10 M

Solution What is the [H3O+] of a solution with a pH of 10.0? STEP 1Enter the pH value, change sign: –10.0 STEP 2Convert pH to concentration: Use 2ndfunction key and then10x key or inverse key and then log key 1−10 STEP 3 Adjust the significant figures in the coefficient (1 digit following decimal point = 1 digit in the coefficient): [H3O+]= 1 x 10−10 M (3)

Learning Check What is the [H3O+] of a solution with a pH of 2.85? 1) 1.0 x 10−2.85 M 2) 1.4 x 10−3 M 3) 8.5 x 10−2 M

Solution What is the [H3O+] of a solution with a pH of 2.85? STEP 1Enter the pH value, change sign: –2.85 STEP 2Convert pH to concentration: Use 2ndfunction key and then10x key or inverse key and then log key 0.0014125 =1.4125 x 10−03 STEP 3 Adjust the significant figures in the coefficient (2 digits following decimal point = 2 digits in the coefficient): [H3O+] = 1.4 x 10−3 M (2)

Chapter 10 Acids and Bases