Basic Chemistry Copyright © 2011 Pearson Education, Inc. 1 Chapter 14 Acids and Bases 14.6 The pH Scale

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 3 pH of Everyday Substances

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 4 Identify each solution as A) acidic, B) basic, or N) neutral. 1. ___ HCl with a pH = ___ pancreatic fluid [H 3 O + ] = 1 x 10 −8 M 3. ___ soft drink pH = ___ pH = ___ [OH − ] = 3 x 10 −10 M 6. ___ [H 3 O + ] = 5 x 10 −12 Learning Check

Basic Chemistry Copyright © 2011 Pearson Education, Inc A)HCl with a pH = B) pancreatic fluid [H 3 O + ] = 1 x 10 −8 M 3. A) soft drink pH = N)pH = A)[OH - ] = 3 x 10 −10 M 6. B) [H 3 O + ] = 5 x 10 −12 M Solution

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 6 Testing the pH of Solutions The pH of solutions is determined by using a pH meter, pH paper, or indicators that have specific colors at different pH values

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 7 Mathematically, pH is the negative logarithm (log) of the hydronium ion concentration. pH = -log [H 3 O + ] For a solution with [H 3 O + ] = 1 x 10 −4 pH =−log [1 x 10  4 ] pH =  [  4.0] pH = 4.0 Calculating pH

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 8 Decimal Places in pH Values When expressing pH values, the number of decimal places is equal to the number of significant figures in the coefficient of [H 3 O + ]. coefficient decimal places [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

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 9 Guide to Calculating pH

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 10 Find the pH of a solution with a [H 3 O + ] of 1.0 x 10 −3. STEP 1 Enter the [H 3 O + ] value. 1.0 [EE or EXP] 3 [+/  ] Display: 1 −03 or 1  03 STEP 2 Press the log key and change the sign. [log (1 x 10 −3 )] [+/  ] = 3 STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient 1.0 x 10 − SFs 2 SFs on the right of the decimal point Example of Calculating pH

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 11 The [H 3 O + ] of tomato juice is 2.2 x 10 −4 M. What is the pH of the solution? 1) 3.7 2) ) Learning Check

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 12 STEP 1 Enter the [H 3 O + ] value. 2.2 [EE or EXP] 4 [+/  ] Display: 2.2 −04 or 2.2  04 STEP 2 Press the log key and change the sign. log[2.2 −04 ] [+/  ] = (Display) STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient 2.2 x 10  (2) 2 SFs 2 SFs on the right of the decimal point Solution

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 13 Calculating pH from [OH − ] Calculate the pH of a solution with [OH − ] of 4.0 x 10 −5. STEP 1 Enter the [H 3 O + ] value K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 = 2.5 x 10 −10 [OH − ] [OH − ] 4.0 x 10 −5 2.5 [EE or EXP]  10 [+/  ] Display: 2.5 −10 or 2.5  10 STEP 2 Press the log key and change the sign.  log[2.5 −10 ] =  [  ] =

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 14 Calculating pH from [OH − ] (continued) STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient. 2.5 x 10 −10 M pH = SFs 2SFs on the right of the decimal point

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 15 pOH The pOH of a solution is analogous to the pH value on the pH scale. is based on the [OH − ] has high pOH values with low [OH − ] and high [H 3 O + ] is equal to the - log [OH − ] pOH = - log [OH − ] added to the pH value is equal to pH + pOH = 14.00

Basic Chemistry Copyright © 2011 Pearson Education, Inc. Example of Calculating pOH Calculate the pOH of a solution that has a pH of pH + pOH = pOH = 14.0 – pH = =

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 17 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 + ] = 10 −pH If the pH is 3.0, the [H 3 O + ] = 1 x 10 −3 M.

Basic Chemistry Copyright © 2011 Pearson Education, Inc. Example of Calculating [H 3 O + ] from pH Calculate the [H 3 O + ] for a pH value of 8.0. [H 3 O + ] = 10 −pH STEP1 Enter the pH value and change the sign. 8.0 [+/  ] Display = –8.0 STEP 2 Convert  pH to [H 3 O + ]. Press the second function key and then the 10 x key or press the inverse key and then the log key. 1 –08 18

Basic Chemistry Copyright © 2011 Pearson Education, Inc. Example of Calculating [H 3 O + ] from pH (continued) STEP 3 Adjust the number of significant figures in the coefficient. Because the pH value of 8.0 has one digit on the right of the decimal point, the [H 3 O + ] is written with one significant figure. For pH = 8.0, [H 3 O + ] = 1 x 10 −8 M 19

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 20 What is the [H 3 O + ] of a solution with a pH of 3.80? Learning Check

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 21 STEP1 Enter the pH value and change the sign [+/  ] Display = –3.80 STEP 2 Convert  pH to [H 3 O + ]. Press the second function key and then the 10 x key or press the inverse key and then the log key (+/-) inv log (or 10 x ) Display =  04 STEP 3 Adjust the number of significant figures in the coefficient. Because the pH value of 3.80 has two digits on the right of the decimal point, the [H 3 O + ] is written with two significant figures in the coefficient. pH 3.80 [H 3 O + ] = 1.6 x 10  4 M Solution

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 22 pH Range

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 23 Learning Check What is the pH and the pOH of coffee if the [H 3 O + ] is 1 x 10 −5 M? 1) pH = 5.0 pOH =7.0 2) pH = 7.0 pOH = 9.0 3) pH = 5.0pOH = 9.0

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 24 Solution STEP 1 Enter the [H 3 O + ] value. 1 [EE or EXP] 5 [+/  ] Display: 1 −05 or 1  05 STEP 2 Press the log key and change the sign. pH = –log [1 x 10 −5 ] = –(–5.0) = 5.0

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 25 Solution (continued) STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient. Because the [H 3 O + ] has one digit in the coefficient, the pH has one digit to the right of the decimal point. pH = 5.0 pH + pOH = pOH = – pH = – 5.0 = 9.0 or [OH − ] = 1 x 10 −9 pOH = –log [1 x 10 −9 ] = –(–9.0) = 9.0 (3)

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 26 What is the [H 3 O + ] of a solution with a pOH of 11.15? 1) 1.0 x 10 −2.85 M 2) 1.4 x 10 −3 M 3) 8.5 x 10 −2 M Learning Check

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 27 STEP1 Enter the pH value and change the sign. pH = – pOH = = [+/  ] =  2.85 STEP2 Convert  pH to [H 3 O + ]. Use second function key and then10 x key or inverse key and then log key. 10  2.85 = x 10 −03 STEP 3 Adjust the significant figures in the coefficient. In the pH value 2.85, there are two digits on the right of the decimal point; the [H 3 O + ] has two significant figures. pH = 2.85 [H 3 O + ] = 1.4 x 10 −3 M (2) Solution